Data Resource Profile: Danish Conscription Registry Data (DCRD)

Data Resource Profile: Danish Conscription Registry Data (DCRD) Data resource basics The physical and mental health of an individual is shaped throughout life, and exposures in early life may influence the risk of later adverse outcomes and survival.1–3 Height, body mass index, cognitive ability and education at the start of adult life are among the traits most consistently associated with later physical and mental health outcomes, as measured by morbidity and mortality. Information on these traits is available in the Danish Conscription Registry Data (DCRD), which consists of conscription data on 1.7 million Danish citizens (99.1% males) examined by the Danish conscription board from 1957 through 2015. Moreover, the data can be linked to individual-level data on socioeconomic and health factors (e.g. educational level, income and diagnosis and date from hospital records) in national registers using the unique personal identification number assigned to all residents of Denmark since 1968. Conscription board examinations have been maintained uninterruptedly since World War II, under the responsibility of the Ministry of Internal Affairs.4 All young Danish men are required by law to have their physical and mental abilities examined by the Danish conscription board, to evaluate their eligibility for military service. Most men are examined when they are 18 or 19 years old, but the examination can be postponed until age 26 upon request.5 Exemption is granted to a small proportion of Danish men who can document an illness or condition (e.g. epilepsy, type I diabetes or intellectual disability) rendering them unfit for military service. Moreover, the few men who volunteer for military service before their 18th birthday are also exempt from the conscription board examination. However, those who are exempt are still registered in the conscription records. In principal, all young Danish men should be registered in the DCRD.5 From 1962, female volunteers have been accepted in the military and from 2006, all women receive an invitation to the conscription board examination when they turn 18.6 However, women still only comprise a minority of those examined (see Table 1). Table 1 Overview of the populations covered and the main variables registered in the four individual sources of conscription data and in the Danish Conscription Registry Data (DCRD) Danish Conscription Database (DCD) National Archives Database (NAD) Danish Defence Personnel Organisation Database (DDPOD) Danish Conscription Register (DCR) Danish Conscription Registry Data (DCRD) Population characteristics  No. of observations 728 159 696 585 219 824 404 808 2 049 376  No. of unique individuals 728 159 658 309 219 824 364 665 1 757 242  Examination data,a % 92.0 75.9 99.6 81.5 83.9  Birth year 1929–60 1940–97 1950–88 1973–98 1929–1998  Examination year 1957–84 1987–2011 1995–2005 2006–15 1957–2015  Sex, % males 100 100b 98.9 96.3 99.1  Age at examination, median (IQR) 19 (18–20) 19 (19–20) 19 (19–20) 19 (19–20) 19 (18–20) Examination variables  Personal identification number (CPR-number) X X X X A  Date of birth X X X X A  Date of examination X Xc X X A  Conscription board district X X X S  Municipality X X S  Profession X X X S  Education X X X S  Height (cm) X X X X A  Weight (kg) Xd X S  Cognitive ability X Xe X X Ae  Hair and eye colour X X S  Colour blindness X X S  Eligibility X X X S  WHO diagnostic codes X S Danish Conscription Database (DCD) National Archives Database (NAD) Danish Defence Personnel Organisation Database (DDPOD) Danish Conscription Register (DCR) Danish Conscription Registry Data (DCRD) Population characteristics  No. of observations 728 159 696 585 219 824 404 808 2 049 376  No. of unique individuals 728 159 658 309 219 824 364 665 1 757 242  Examination data,a % 92.0 75.9 99.6 81.5 83.9  Birth year 1929–60 1940–97 1950–88 1973–98 1929–1998  Examination year 1957–84 1987–2011 1995–2005 2006–15 1957–2015  Sex, % males 100 100b 98.9 96.3 99.1  Age at examination, median (IQR) 19 (18–20) 19 (19–20) 19 (19–20) 19 (19–20) 19 (18–20) Examination variables  Personal identification number (CPR-number) X X X X A  Date of birth X X X X A  Date of examination X Xc X X A  Conscription board district X X X S  Municipality X X S  Profession X X X S  Education X X X S  Height (cm) X X X X A  Weight (kg) Xd X S  Cognitive ability X Xe X X Ae  Hair and eye colour X X S  Colour blindness X X S  Eligibility X X X S  WHO diagnostic codes X S X indicates if a variable is present in the individual source of conscription data; A indicates if a variable is present in all four sources; and S indicates if a variable is present in a subset of the four individual sources of conscription data. NA, not available, IQR, interquartile range. a Data on cognitive ability are used as a proxy for presence of examination data. b NAD includes 25 women. c In NAD: a large proportion (14.8%) have missing information on date of examination because they have been exempted from the conscription board examination. d In DCD: weight was not recorded in all conscription districts—see www.regionh.dk/dcd for details on the distribution of weight assessments. e In NAD: cognitive ability was only reported as a categorical variable with the categories 1–5 where category 5 included everyone with a score above the mean. Table 1 Overview of the populations covered and the main variables registered in the four individual sources of conscription data and in the Danish Conscription Registry Data (DCRD) Danish Conscription Database (DCD) National Archives Database (NAD) Danish Defence Personnel Organisation Database (DDPOD) Danish Conscription Register (DCR) Danish Conscription Registry Data (DCRD) Population characteristics  No. of observations 728 159 696 585 219 824 404 808 2 049 376  No. of unique individuals 728 159 658 309 219 824 364 665 1 757 242  Examination data,a % 92.0 75.9 99.6 81.5 83.9  Birth year 1929–60 1940–97 1950–88 1973–98 1929–1998  Examination year 1957–84 1987–2011 1995–2005 2006–15 1957–2015  Sex, % males 100 100b 98.9 96.3 99.1  Age at examination, median (IQR) 19 (18–20) 19 (19–20) 19 (19–20) 19 (19–20) 19 (18–20) Examination variables  Personal identification number (CPR-number) X X X X A  Date of birth X X X X A  Date of examination X Xc X X A  Conscription board district X X X S  Municipality X X S  Profession X X X S  Education X X X S  Height (cm) X X X X A  Weight (kg) Xd X S  Cognitive ability X Xe X X Ae  Hair and eye colour X X S  Colour blindness X X S  Eligibility X X X S  WHO diagnostic codes X S Danish Conscription Database (DCD) National Archives Database (NAD) Danish Defence Personnel Organisation Database (DDPOD) Danish Conscription Register (DCR) Danish Conscription Registry Data (DCRD) Population characteristics  No. of observations 728 159 696 585 219 824 404 808 2 049 376  No. of unique individuals 728 159 658 309 219 824 364 665 1 757 242  Examination data,a % 92.0 75.9 99.6 81.5 83.9  Birth year 1929–60 1940–97 1950–88 1973–98 1929–1998  Examination year 1957–84 1987–2011 1995–2005 2006–15 1957–2015  Sex, % males 100 100b 98.9 96.3 99.1  Age at examination, median (IQR) 19 (18–20) 19 (19–20) 19 (19–20) 19 (19–20) 19 (18–20) Examination variables  Personal identification number (CPR-number) X X X X A  Date of birth X X X X A  Date of examination X Xc X X A  Conscription board district X X X S  Municipality X X S  Profession X X X S  Education X X X S  Height (cm) X X X X A  Weight (kg) Xd X S  Cognitive ability X Xe X X Ae  Hair and eye colour X X S  Colour blindness X X S  Eligibility X X X S  WHO diagnostic codes X S X indicates if a variable is present in the individual source of conscription data; A indicates if a variable is present in all four sources; and S indicates if a variable is present in a subset of the four individual sources of conscription data. NA, not available, IQR, interquartile range. a Data on cognitive ability are used as a proxy for presence of examination data. b NAD includes 25 women. c In NAD: a large proportion (14.8%) have missing information on date of examination because they have been exempted from the conscription board examination. d In DCD: weight was not recorded in all conscription districts—see www.regionh.dk/dcd for details on the distribution of weight assessments. e In NAD: cognitive ability was only reported as a categorical variable with the categories 1–5 where category 5 included everyone with a score above the mean. The conscription board examination consists of a questionnaire, an intelligence test, and a health examination and is conducted according to national guidelines. After the examination, the conscription board evaluates whether the conscript is fit for military service, for limited duties or unfit for military service. From 1957, the collected data were recorded in a standardized format on register cards and in 1987, the handling and storage of data were digitized according to the terms and regulations of the Danish Data Protection Agency.5 Today conscription board data are available from the following four sources: the Danish Conscription Database (DCD), the National Archives Database (NAD), the Danish Defence Personnel Organisation Database (DDPOD) and the Danish Conscription Register (DCR). For the period from roughly 1957 through 1977, conscription board data are available in the DCD, which was established for research purposes through manual digitization of conscription board register cards. The Danish Defence Personnel Organisation has digitally recorded key data from all conscription board examinations since 1987. The three other sources are based on these electronic registrations, leaving a gap of almost 10 years (from 1978 through 1986) for which data have not been digitalized. Data collected from 1987 through 2005 have been stored in two separate databases, the NAD and the DDPOD, with a considerable overlap of individuals but differences in the selected variables. The NAD consists of a copy of selected variables for all individuals examined in the period from 1987 through 2011, which has been transferred from the Danish Defence Personnel Organisation to the Danish National Archives. The DDPOD consists of another copy of more detailed variables for individuals examined from 1995 through 2005. This copy has been stored by the Danish Defence Personnel Organisation and predominantly covers men and women who were evaluated as fit for military service or limited duties. Since 2006, a copy of all data collected at the Danish conscription board examinations has been transferred from the conscription authorities to the DCR, which is an official register in the Danish Health Data Authority. As of March 2018 the Danish Health Data Authority has decided to transfer the responsibility of the DCR to the Danish Defence Personnel Organisation, which will handle future updates of the register. The DCRD consists of pooled conscription data from the four sources and provides more than 1.7 million individual observations on men and women born predominantly from 1939 through 1998, who appeared at the conscription board examination from 1957 through 2015. Data collected Coverage Table 1 gives an overview of the main characteristics of the populations covered in the four individual sources of conscription data and combined in the DCRD. The Danish Conscription Database (DCD) The DCD is a database with conscription board examination data for 728 159 men born between 1929 and 1960 (99.8% born between 1939 and 1959) and examined from 1957 through 1984 (99.8% examined between 1958 and 1978). Information on selected variables was digitized and, in case of multiple records on a single individual, the first record containing information on cognitive ability was chosen. For more details on the establishment of the DCD, see Christensen et al. 2015.7 The National Archives Database (NAD) The NAD contains 696 585 observations with data on 658 309 unique individuals of whom only 25 were women. The population was born from 1940 through 1997 (99.8% born between 1969 and 1986) and examined at the conscription board from 1987 through 2011 (83.8% examined between 1988 and 2005). The Danish Defence Personnel Organisation Database (DDPOD) The DDPOD contains conscription data on 219 824 unique individuals (98.9% males) evaluated as fit for military service or limited duties. They were born from 1950 through 1988 (90.7% born between 1976 and 1986) and examined from 1995 through 2005. The Danish Conscription Register (DCR) The DCR is a continuously updated register of conscription data for all men and women examined at the conscription board from 2006 and onwards. The population described in this paper contains conscription data for all men and women examined from 2006 through 2015. The DCR contains 404 808 observations with data on 364.665 unique individuals of whom 96.3% were men. They were born from 1973 through 1998 (97.1% born between 1986 and 1997). Figure 1 presents an overview of the birth cohorts covered by the four sources of conscription board data. The figure clearly illustrates the gap between the DCD and NAD and the overlap between the NAD and DDPOD. The approximate time of conscription board examination is marked by the horizontal line at age 19 (median age at examination). In 2018, the oldest individuals in the DCRD will be 80 years old and the youngest will be 20 years old. The first vertical line on the left represents the introduction of personal identification numbers (CPR-numbers) on 2 April 1968. Figure 1 View largeDownload slide Overview of the follow-up periods for the four sources of conscription board data and the timing of selected national health registers. Sources: DCD: Danish Conscription Database, NAD: National Archives Database, DDPOD: Danish Defence Personal Organisation Database, DCR: Danish Conscription Register. Registers: DRCD: Danish Register of Cause of Death, MBR: Medical Birth Register, DNPR: Danish National Patient Register, NPR: National Prescription Register. CPR-number: Central Person Registration number (personal identification number). Figure 1 View largeDownload slide Overview of the follow-up periods for the four sources of conscription board data and the timing of selected national health registers. Sources: DCD: Danish Conscription Database, NAD: National Archives Database, DDPOD: Danish Defence Personal Organisation Database, DCR: Danish Conscription Register. Registers: DRCD: Danish Register of Cause of Death, MBR: Medical Birth Register, DNPR: Danish National Patient Register, NPR: National Prescription Register. CPR-number: Central Person Registration number (personal identification number). The Danish Conscription Board Data (DCRD) Combining the four samples, the DCRD consists of more than 1.7 million unique individuals examined at the conscription board from 1957 through 2015. The median age at conscription is 19 years (Table 1). Most men (86.0%) are examined between the ages of 18 and 20, but a few (12.8%) have their examination postponed and are examined between the ages of 21 and 26. Men who volunteer before the age of 18 are exempted from appearing before the conscription board but they are still registered, and 0.9% of the men in the DCRD are 17 years old. Moreover, if conscripted, the military service can under special circumstances be postponed until age 32 and a very few (0.3%) are thus re-examined between the ages of 27 and 32. The proportion of individuals for whom examination data (estimated as individuals with data on cognitive ability) are available ranges from around 75% to around 93% and is highest for those born before 1959 and lowest for those born from 1970 through 1979 (Table 1). The proportion with missing examination data mainly represent those exempted as previously described. Given that both those examined and those exempted are registered, the DCRD should cover all Danish men born from 1939 through 1959 and from 1969 through 1997, who have survived until age 18, with the exception of the few who died before being examined. To assess to what degree the DCRD covers the male population residing in Denmark, we divided the DCRD population into 10-year birth cohorts and compared the number of males in the DCRD with the estimated number of 24-year-old males registered in Statistics Denmark (see Table 2). The 24-year-olds were chosen as the reference because the younger age groups (the 18–23 year olds) are characterized by a high level of migration which complicates the comparison and because the number of males is only available for 5-year age groups from age 25 and onwards. Table 2 illustrates how the Danish male population is covered in the DCRD, and coverage is high in all birth cohorts except for those born from 1960 through 1969. This gap represents the gap between the DCD and NAD. Data on these men are available on register cards and are stored in the Danish National Archives. Table 2 Data coverage assessed through comparison of the Danish Conscription Registry Data (DCRD) population and number of males in the corresponding birth cohorts in the Danish population Birth cohort Total 1939–49 1950–59 1960–69 1970–79 1980–89 1990–97 Number of Danish males alive at age 24a 521 963 383 948 387 561 406 013 332 935 279 098 2 311 518 Number of males in DCRD 393 645 333 360 45 596 417 038 308 390 243 042 1 741 071 Proportion (%) of birth cohort covered in DCRD 75.4 86.8 11.8 102.7 92.6 87.1 75.3 Number of males having examination data 364 858 303 862 37 954 313 591 244 707 194 954 1 460 033 Proportion (%) of DCRD having examination datab 92.7 91.2 83.2 75.2 79.3 80.2 83, 9 Birth cohort Total 1939–49 1950–59 1960–69 1970–79 1980–89 1990–97 Number of Danish males alive at age 24a 521 963 383 948 387 561 406 013 332 935 279 098 2 311 518 Number of males in DCRD 393 645 333 360 45 596 417 038 308 390 243 042 1 741 071 Proportion (%) of birth cohort covered in DCRD 75.4 86.8 11.8 102.7 92.6 87.1 75.3 Number of males having examination data 364 858 303 862 37 954 313 591 244 707 194 954 1 460 033 Proportion (%) of DCRD having examination datab 92.7 91.2 83.2 75.2 79.3 80.2 83, 9 a Number of Danish males are derived from Statistics Denmark. The number of males alive at age 26, the age limit for postponement of the conscription board examination, are not available (numbers are grouped in 5-year intervals from age 25 and onwards) and age 24 was thus chosen instead. b Data on cognitive ability are used as a proxy for presence of examination data. Table 2 Data coverage assessed through comparison of the Danish Conscription Registry Data (DCRD) population and number of males in the corresponding birth cohorts in the Danish population Birth cohort Total 1939–49 1950–59 1960–69 1970–79 1980–89 1990–97 Number of Danish males alive at age 24a 521 963 383 948 387 561 406 013 332 935 279 098 2 311 518 Number of males in DCRD 393 645 333 360 45 596 417 038 308 390 243 042 1 741 071 Proportion (%) of birth cohort covered in DCRD 75.4 86.8 11.8 102.7 92.6 87.1 75.3 Number of males having examination data 364 858 303 862 37 954 313 591 244 707 194 954 1 460 033 Proportion (%) of DCRD having examination datab 92.7 91.2 83.2 75.2 79.3 80.2 83, 9 Birth cohort Total 1939–49 1950–59 1960–69 1970–79 1980–89 1990–97 Number of Danish males alive at age 24a 521 963 383 948 387 561 406 013 332 935 279 098 2 311 518 Number of males in DCRD 393 645 333 360 45 596 417 038 308 390 243 042 1 741 071 Proportion (%) of birth cohort covered in DCRD 75.4 86.8 11.8 102.7 92.6 87.1 75.3 Number of males having examination data 364 858 303 862 37 954 313 591 244 707 194 954 1 460 033 Proportion (%) of DCRD having examination datab 92.7 91.2 83.2 75.2 79.3 80.2 83, 9 a Number of Danish males are derived from Statistics Denmark. The number of males alive at age 26, the age limit for postponement of the conscription board examination, are not available (numbers are grouped in 5-year intervals from age 25 and onwards) and age 24 was thus chosen instead. b Data on cognitive ability are used as a proxy for presence of examination data. Data contents The variables included A number of variables are present in all four sources (Table 1). A core variable is the unique personal identification number, the Danish Central Person Register (CPR) number. This 10-digit number combines birth date (in the format DDMMYY), and a sequence number of four digits from which the century of birth and sex of the person can be derived. The sources also include information on the time of conscription board examination. In the DCD, the year and half-year of the examination are registered whereas an examination date, which is registered within 2 weeks of the date of the actual examination, is available in the NAD, DDPOD and DCR. Moreover, information on height and cognitive ability are also present in all four sources. Height (in cm) is measured without shoes and to the nearest cm using a stadiometer. Cognitive ability has been assessed using an intelligence test called Børge Prien’s Prøve (BPP), named after Børge Prien the military psychologist who developed the test. The BPP is a group-administered test consisting of four subtests to be completed within 45 min: (i) letter matrices; (ii) verbal analogies; (iii) number series; and (iv) geometrical figures.4 The BPP consists of 78 items that have remained unchanged from 1957 to the present day, but the test was adapted to a computerized format at the end of 2010. Table 1 also gives an overview of other variables which are present in a subset of the four sources of conscription data. Information on education is available in the DCD, DDPOD and DCR. The men and women examined report their current level of education to the conscription board. From 1957 through 1978, education was coded on a scale from 1 to 9 and reflected a combination of formal schooling and vocational training (for more details see Teasdale and Owen 19868). From 1979 and onwards, education has been coded on a scale from 8 to 13 and reflected number of years of formal schooling. Body weight is only registered in the DCD and DCR, and only the DCR includes information on medical conditions classified according to the international classification of diseases (ICD) system. Six months before the examination, the men and women are required to fill in an electronic form which includes questions on health (e.g. questions on current and previous medical conditions (somatic and psychiatric). A conscription board physician evaluates this information before performing a medical examination at the conscription board examination. Documentation of the individual data sources is available on request. Data quality The conscription board examinations have always been carried out according to national guidelines, by trained medical doctors using standardized methods for measurement of height and weight. As described, the intelligence test (BPP) has also been used without change since 1957 and the total BPP score has been shown to have a substantial correlation(r = 0.82) with the full-scale Wechsler Adult Intelligence Scale.9 As mentioned, the DCD was established through manual digitization of conscription board register cards and, to ensure data quality, 1% of all cards were entered twice by two different people and subsequently compared. The error rates ranged from 0.5% to 1.4%, depending on the variable. Given that around 30% of the DCD men were examined before the introduction of personal identification numbers (CPR-numbers) in 1968, the CPR numbers were traced by matching on name and birth date. This can potentially have led to error in matching, but 88.0% of the DCD men were matched directly (exact same name and birth date, with no difference in spelling of the name) and a smaller percentage of the matched individuals were manually examined, and there was no indication of systematic errors. Data resource use The four sources of conscription data include anthropometric measures and cognitive ability assessed in young adulthood and, due to the unique personal identification number, it is possible to link these data to a large number of Danish health and social registers or cohort studies. The potential of linking the DCRD as well as each individual source of conscription board data to Danish registers is illustrated in Figure 1 by the vertical lines representing the start year of a selection of national health registers. The first line from the left shows the introduction of the personal identification numbers in 1968, which has enabled the linkage of all Danish registries. Until now all four nationwide sources of conscription data have not been used in pooled analysis. DCD has been used as a stand-alone resource in national and international studies of time trends in height10 and BMI.11 It has also been linked to health registers and employed in studies of the impact of cognitive ability on cause-specific mortality and dementia.12,13 In ongoing studies, the DCD is linked to the Copenhagen Aging and Midlife Biobank14 and the Copenhagen School Health Records Register15 in order to study whether changes in overweight over the life course influence later health. The DCR has also been used as a stand-alone resource in studies on trends in BMI16 and has been linked to health registers, e.g. in studies of the impact of infections on cognitive ability in men.17 The DCD, DDPOD and DCR have been used in combined analyses in two completed and several ongoing studies. In a study on the potential bi-directional association between cognitive ability and epilepsy, the authors found that epilepsy (hospital contacts and anti-epileptic treatment) before conscription board examination was associated with lower cognitive ability, and that low cognitive ability at examination was associated with a higher risk of epilepsy later in life.18 The authors thus took advantage of the fact that the youngest individuals in the DCRD could be followed in health registers for information on epilepsy before the examination, and the oldest could be followed after the examination. The conscription board data from the DCD, DDPOD and DCR have also been linked with self-reported data on psychological reactions after returning home from military deployments. In one study the authors found an inverse association between cognitive ability at conscription and post-deployment symptoms of post-traumatic stress disorder,19 and another paper examining the association between cognitive ability and depression is in the pipeline. The possibility to identify family relations for individuals born after 1955, given the unique personal identification number, represents an extremely promising opportunity to study family relations. Table 3 provides an example of such analyses based on the pooled data from the DCD, NAD, DDPOD and DCR that together constitutes the DCRD. It shows that the increasing trend in men’s mean height seemed to level off for birth cohorts born after 1980, and within families the second-born brother seemed to be taller than the first-born during the entire time period under study. Table 3 Mean height and differences in height between first- and second-born brothers across birth cohorts for men in the Danish Conscription Registry Data (DCRD) Birth cohort Total 1939–49 1950–59 1960–69 1970–79 1980–89 1990–97 Number of males in DCRD 393 645 333 360 45 596 417 038 308 390 243 042 1 741 071 Mean height (cm) 175.8 178.0 178.8 180.1 180.2 180.5 178.3 Number (%) with maternal ID 40 946 225 227 39 473 362 672 290 409 242 425 1 200 122 (10.4) (67.6) (86.5) (86.7) (94.2) (99.8) (68.9) Numbers (%) with at least one brother 10 707 74 282 14 565 170 971 164 442 112 048 547 014 (2.7) (22.3) (31.9) (41.0) (53.3) (46.1) (31.4) Mean [SD] difference in height (cm) between second- and first-born brother 0.46 0.43 0.68 0.02 0.18 0.59 0.37 (6.2) (6.5) (6.6) (6.5) (7.6) (7.1) (7.2) Birth cohort Total 1939–49 1950–59 1960–69 1970–79 1980–89 1990–97 Number of males in DCRD 393 645 333 360 45 596 417 038 308 390 243 042 1 741 071 Mean height (cm) 175.8 178.0 178.8 180.1 180.2 180.5 178.3 Number (%) with maternal ID 40 946 225 227 39 473 362 672 290 409 242 425 1 200 122 (10.4) (67.6) (86.5) (86.7) (94.2) (99.8) (68.9) Numbers (%) with at least one brother 10 707 74 282 14 565 170 971 164 442 112 048 547 014 (2.7) (22.3) (31.9) (41.0) (53.3) (46.1) (31.4) Mean [SD] difference in height (cm) between second- and first-born brother 0.46 0.43 0.68 0.02 0.18 0.59 0.37 (6.2) (6.5) (6.6) (6.5) (7.6) (7.1) (7.2) Table 3 Mean height and differences in height between first- and second-born brothers across birth cohorts for men in the Danish Conscription Registry Data (DCRD) Birth cohort Total 1939–49 1950–59 1960–69 1970–79 1980–89 1990–97 Number of males in DCRD 393 645 333 360 45 596 417 038 308 390 243 042 1 741 071 Mean height (cm) 175.8 178.0 178.8 180.1 180.2 180.5 178.3 Number (%) with maternal ID 40 946 225 227 39 473 362 672 290 409 242 425 1 200 122 (10.4) (67.6) (86.5) (86.7) (94.2) (99.8) (68.9) Numbers (%) with at least one brother 10 707 74 282 14 565 170 971 164 442 112 048 547 014 (2.7) (22.3) (31.9) (41.0) (53.3) (46.1) (31.4) Mean [SD] difference in height (cm) between second- and first-born brother 0.46 0.43 0.68 0.02 0.18 0.59 0.37 (6.2) (6.5) (6.6) (6.5) (7.6) (7.1) (7.2) Birth cohort Total 1939–49 1950–59 1960–69 1970–79 1980–89 1990–97 Number of males in DCRD 393 645 333 360 45 596 417 038 308 390 243 042 1 741 071 Mean height (cm) 175.8 178.0 178.8 180.1 180.2 180.5 178.3 Number (%) with maternal ID 40 946 225 227 39 473 362 672 290 409 242 425 1 200 122 (10.4) (67.6) (86.5) (86.7) (94.2) (99.8) (68.9) Numbers (%) with at least one brother 10 707 74 282 14 565 170 971 164 442 112 048 547 014 (2.7) (22.3) (31.9) (41.0) (53.3) (46.1) (31.4) Mean [SD] difference in height (cm) between second- and first-born brother 0.46 0.43 0.68 0.02 0.18 0.59 0.37 (6.2) (6.5) (6.6) (6.5) (7.6) (7.1) (7.2) Strengths and weaknesses The DCRD covers a considerable proportion of Danish men born from 1939 through 1959 and from 1969 onwards. Whereas conscription is mandatory for men, which ensures a high level of generalizability of the male DCRD population, the few women in the DCRD represent a selected group who have volunteered for military service. Only relatively few variables of interest for health research are included. However, the information has been collected by trained medical doctors using standardized methods. The coverage or coding of some of the relevant variables, such as weight and cognitive ability, varies across the four sources of conscription data. However, in combination with data from Danish registers, measures of young adult physical and mental characteristics can be used as exposures, covariables and outcomes in epidemiological studies of health across the life course. The DCD differs from the other three sources of data given that it was established through manual digitization and only one record per person was digitized. Furthermore, the DDPOD only contains one record per person and the population is for the most part restricted to those who were evaluated as fit for military service or limited duties. Consequently, the individuals having the poorest scores on the individual variables have been excluded. Even though the DDPOD population is also present in the NAD, the DDPOD is included because of better information on cognitive ability compared with the NAD. Data resource access Access to the Danish Conscription Register Data including personal identification numbers can only be obtained with approval from the Danish Data Protection Agency [www.datatilsynet.dk]. Researchers can contact the individual sources of conscription data with their project description and approval as follows. The DCD can be contacted through the homepage [www.regionh.dk/dcd]. Data from the NAD and DCR can be applied for by filling out a form found on the websites of the National Danish Archives [https://www.sa.dk/brug-arkivet/bestil/digitale-arkivalier] or the Danish Health Data Authority [http://sundhedsdatastyrelsen.dk/da/forskerservice/ansoegningsskema/ansoegningsskema-fsk]. As the Danish Health Data Authority has decided to transfer the responsibility of the DCR to the Danish Defence Personnel Organisation, requests for access to DCR will in the future be managed by the Danish Defence Personnel Organisation. Data from the DDPOD can be obtained by submission of an application to the Danish Defence Personnel Organisation. Profile in a nutshell The DCRD was established to take advantage of the unique data on physical and mental characteristics of all young Danish men collected at the Danish conscription board examinations. The DCRD consists of conscription data on 1.7 million Danish citizens (99.1% males) examined at the Danish conscription board from 1957 through 2015 and covers around 75% of Danish men born from 1939 through 1997. By law all Danish men, with the exemption of those having a serious medical condition (e.g. epilepsy, type I diabetes or intellectual disability), are required to appear before the conscription board between ages of 18 to 26. From 1962, women volunteers have been accepted in the military but still remain a minority (0.9% in DCRD). The conscription board examination is carried out according to standardized national guidelines and consists of a questionnaire, an intelligence test and a physical examination. The data collected have been recorded in a standardized format, with few changes from 1957 up to today. The main categories of data collected are personal identification number, sex, birth date, date of conscription examination, height, education and cognitive ability. Moreover, the data can be linked to all Danish registry data using the personal identification number. Access to DCRD including personal identification numbers requires approval from the Danish Data Inspection Agency. For each source of data, access can be applied for as follows: the DCD—contact the steering group through [www.region.dk/dcd]; the NAD—contact the National Danish Archives [www.sa.dk]; DDPOD—contact Danish Defence Personnel Organisation [www.forpers.dk]; DCR—contact the Danish Health Data Authority [www.sundhedsdatastyrelsen.dk]. Funding This work was mainly supported by the Danish Medical Research Council (grant numbers 09–063599 and 09–069151), the Velux Foundation (grant number 95–103-11419), the Jascha Foundation (grant number 6200) and Doctor Sofus Carl Emil Friis and Olga Doris Friis grant. Conflict of interest: The authors declare no conflict of interest. Author Contributions M.O. conceived the study idea. G.T.C., M.O. and S.S. made the analyses and wrote the initial draft. All authors contributed to critical revision of the paper and agreed to be accountable for all aspects of the work. References 1 Lynch J , Davey Smith G. A life course approach to chronic disease epidemiology . Ann Rev Public Health 2005 ; 26 : 1 – 35 . Google Scholar CrossRef Search ADS 2 Kuh D , Ben-Shlomo Y , Lynch J , Hallqvist J , Power C. Life course epidemiology . J Epidemiol Community Health 2003 ; 57 : 778 – 83 . Google Scholar CrossRef Search ADS PubMed 3 Cole M , Cole SR , Lightfood C. The Development of Children , 5th edn . New York, NY : Worth Publishers , 2005 . 4 Teasdale TW. The Danish draft board’s intelligence test, Borge Priens Prove: psychometric properties and research applications through 50 years . Scand J Psychol 2009 ; 50 : 633 – 38 . Google Scholar CrossRef Search ADS PubMed 5 Green A. The Danish Conscription Registry: a resource for epidemiological research . Dan Med Bull 1996 ; 43 : 464 – 67 . Google Scholar PubMed 6 The Danish Defence. Danish Defence—Information on Diversity. 2016 . https://www2.forsvaret.dk/temaer/mangfoldighed/Documents/Historisk%20tidslinie%20-%20Ligestilling%20og%20mangfoldighed%20i%20Forsvaret.pdf (7 September 2017, date last accessed). 7 Christensen GT , Molbo D , Angquist LH , Mortensen EL , Christensen K , Sorensen TI. Cohort Profile: The Danish Conscription Database(DCD): a cohort of 728 160 men born from 1939 through 1959 . Int J Epidemiol 2015 ; 44 : 432 – 40 . Google Scholar CrossRef Search ADS PubMed 8 Teasdale TW , Owen DR. The influence of paternal social class on intelligence and educational level in male adoptees and non-adoptees . Br J Educ Psychol 1986 ; 56 : 3 – 12 . Google Scholar CrossRef Search ADS 9 Mortensen EL , Reinisch JM , Teasdale TW. Intelligence as measured by the WAIS and a military draft board group test . Scand J Psychol 1989 ; 30 : 315 – 18 . Google Scholar CrossRef Search ADS 10 NCD Risk factor Collaboration (NCD-RisC) . A century of trends in adult human height . eLife 2016 ; 5 : e13410. PubMed 11 NCD Risk factor Collaboration (NCD-RisC) . Trends in adult body-mass index in 200 countries from 1975 to 2014: a pooled analysis of 1698 population-based measurement studies with 19.2 million participants . Lancet 2016 ; 387 : 1377 – 96 . CrossRef Search ADS PubMed 12 Christensen GT , Mortensen EL , Christensen K , Osler M. Intelligence in young adulthood and cause-specific mortality in the Danish Conscription Database—a Cohort study of 728, 160 men . Intelligence 2016 ; 59 : 64 – 71 . Google Scholar CrossRef Search ADS 13 Osler M , Christensen GT , Garde E , Mortensen EL , Christensen K. Cognitive ability in young adulthood and risk of dementia in a cohort of Danish men, brothers, and twins . Alzheimers Dement 2017 ; 13 : 1355 – 63 . Google Scholar CrossRef Search ADS PubMed 14 Pedersen JM , Budtz-Jorgensen E , Mortensen EL et al. Late midlife C-reactive protein and interleukin-6 in middle aged danish men in relation to body size history within and across generations . Obesity (Silver Spring) 2016 ; 24 : 461 – 68 . Google Scholar CrossRef Search ADS PubMed 15 Baker JL , Olsen LW , Andersen I , Pearson S , Hansen B , Sorensen TI. Cohort Profile: The Copenhagen School Health Records Register . Int J Epidemiol 2009 ; 38 : 656 – 62 . Google Scholar CrossRef Search ADS PubMed 16 NCD Risk factor Collaboration (NCD-RisC) . Worldwide trends in body-mass index, underweight, overweight, and obesity from 1975 to 2016: a pooled analysis of 2416 population-based measurement studies in 128.9 million children, adolescents, and adults . Lancet 2017 ; 390 : 2627 – 42 . CrossRef Search ADS PubMed 17 Benros ME , Sorensen HJ , Nielsen PR , Nordentoft M , Mortensen PB , Petersen L. The association between infections and general cognitive ability in young men—a Nationwide Study . PLoS One 2015 ; 10 : e0124005. Google Scholar CrossRef Search ADS PubMed 18 Osler M , Christensen GT , Christensen K , Mortensen EL. A bidirectional association between cognitive ability in young adulthood and epilepsy: a population-based cohort study . Int J Epidemiol 2018 , Feb 14. doi: 10.1093/ije/dyy018. 19 Nissen LR , Karstoft K-I , Vedtofte MS , Nielsen ABS , Osler M , Mortensen EL. Cognitive ability and risk of post-traumatic stress disorder after military deployment: an observational cohort study . BJPsych Open 2017 ; 3 : 274 – 80 . Google Scholar CrossRef Search ADS PubMed © The Author(s) 2018; all rights reserved. Published by Oxford University Press on behalf of the International Epidemiological Association This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png International Journal of Epidemiology Oxford University Press

Data Resource Profile: Danish Conscription Registry Data (DCRD)

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Oxford University Press
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© The Author(s) 2018; all rights reserved. Published by Oxford University Press on behalf of the International Epidemiological Association
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0300-5771
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1464-3685
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10.1093/ije/dyy048
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Abstract

Data resource basics The physical and mental health of an individual is shaped throughout life, and exposures in early life may influence the risk of later adverse outcomes and survival.1–3 Height, body mass index, cognitive ability and education at the start of adult life are among the traits most consistently associated with later physical and mental health outcomes, as measured by morbidity and mortality. Information on these traits is available in the Danish Conscription Registry Data (DCRD), which consists of conscription data on 1.7 million Danish citizens (99.1% males) examined by the Danish conscription board from 1957 through 2015. Moreover, the data can be linked to individual-level data on socioeconomic and health factors (e.g. educational level, income and diagnosis and date from hospital records) in national registers using the unique personal identification number assigned to all residents of Denmark since 1968. Conscription board examinations have been maintained uninterruptedly since World War II, under the responsibility of the Ministry of Internal Affairs.4 All young Danish men are required by law to have their physical and mental abilities examined by the Danish conscription board, to evaluate their eligibility for military service. Most men are examined when they are 18 or 19 years old, but the examination can be postponed until age 26 upon request.5 Exemption is granted to a small proportion of Danish men who can document an illness or condition (e.g. epilepsy, type I diabetes or intellectual disability) rendering them unfit for military service. Moreover, the few men who volunteer for military service before their 18th birthday are also exempt from the conscription board examination. However, those who are exempt are still registered in the conscription records. In principal, all young Danish men should be registered in the DCRD.5 From 1962, female volunteers have been accepted in the military and from 2006, all women receive an invitation to the conscription board examination when they turn 18.6 However, women still only comprise a minority of those examined (see Table 1). Table 1 Overview of the populations covered and the main variables registered in the four individual sources of conscription data and in the Danish Conscription Registry Data (DCRD) Danish Conscription Database (DCD) National Archives Database (NAD) Danish Defence Personnel Organisation Database (DDPOD) Danish Conscription Register (DCR) Danish Conscription Registry Data (DCRD) Population characteristics  No. of observations 728 159 696 585 219 824 404 808 2 049 376  No. of unique individuals 728 159 658 309 219 824 364 665 1 757 242  Examination data,a % 92.0 75.9 99.6 81.5 83.9  Birth year 1929–60 1940–97 1950–88 1973–98 1929–1998  Examination year 1957–84 1987–2011 1995–2005 2006–15 1957–2015  Sex, % males 100 100b 98.9 96.3 99.1  Age at examination, median (IQR) 19 (18–20) 19 (19–20) 19 (19–20) 19 (19–20) 19 (18–20) Examination variables  Personal identification number (CPR-number) X X X X A  Date of birth X X X X A  Date of examination X Xc X X A  Conscription board district X X X S  Municipality X X S  Profession X X X S  Education X X X S  Height (cm) X X X X A  Weight (kg) Xd X S  Cognitive ability X Xe X X Ae  Hair and eye colour X X S  Colour blindness X X S  Eligibility X X X S  WHO diagnostic codes X S Danish Conscription Database (DCD) National Archives Database (NAD) Danish Defence Personnel Organisation Database (DDPOD) Danish Conscription Register (DCR) Danish Conscription Registry Data (DCRD) Population characteristics  No. of observations 728 159 696 585 219 824 404 808 2 049 376  No. of unique individuals 728 159 658 309 219 824 364 665 1 757 242  Examination data,a % 92.0 75.9 99.6 81.5 83.9  Birth year 1929–60 1940–97 1950–88 1973–98 1929–1998  Examination year 1957–84 1987–2011 1995–2005 2006–15 1957–2015  Sex, % males 100 100b 98.9 96.3 99.1  Age at examination, median (IQR) 19 (18–20) 19 (19–20) 19 (19–20) 19 (19–20) 19 (18–20) Examination variables  Personal identification number (CPR-number) X X X X A  Date of birth X X X X A  Date of examination X Xc X X A  Conscription board district X X X S  Municipality X X S  Profession X X X S  Education X X X S  Height (cm) X X X X A  Weight (kg) Xd X S  Cognitive ability X Xe X X Ae  Hair and eye colour X X S  Colour blindness X X S  Eligibility X X X S  WHO diagnostic codes X S X indicates if a variable is present in the individual source of conscription data; A indicates if a variable is present in all four sources; and S indicates if a variable is present in a subset of the four individual sources of conscription data. NA, not available, IQR, interquartile range. a Data on cognitive ability are used as a proxy for presence of examination data. b NAD includes 25 women. c In NAD: a large proportion (14.8%) have missing information on date of examination because they have been exempted from the conscription board examination. d In DCD: weight was not recorded in all conscription districts—see www.regionh.dk/dcd for details on the distribution of weight assessments. e In NAD: cognitive ability was only reported as a categorical variable with the categories 1–5 where category 5 included everyone with a score above the mean. Table 1 Overview of the populations covered and the main variables registered in the four individual sources of conscription data and in the Danish Conscription Registry Data (DCRD) Danish Conscription Database (DCD) National Archives Database (NAD) Danish Defence Personnel Organisation Database (DDPOD) Danish Conscription Register (DCR) Danish Conscription Registry Data (DCRD) Population characteristics  No. of observations 728 159 696 585 219 824 404 808 2 049 376  No. of unique individuals 728 159 658 309 219 824 364 665 1 757 242  Examination data,a % 92.0 75.9 99.6 81.5 83.9  Birth year 1929–60 1940–97 1950–88 1973–98 1929–1998  Examination year 1957–84 1987–2011 1995–2005 2006–15 1957–2015  Sex, % males 100 100b 98.9 96.3 99.1  Age at examination, median (IQR) 19 (18–20) 19 (19–20) 19 (19–20) 19 (19–20) 19 (18–20) Examination variables  Personal identification number (CPR-number) X X X X A  Date of birth X X X X A  Date of examination X Xc X X A  Conscription board district X X X S  Municipality X X S  Profession X X X S  Education X X X S  Height (cm) X X X X A  Weight (kg) Xd X S  Cognitive ability X Xe X X Ae  Hair and eye colour X X S  Colour blindness X X S  Eligibility X X X S  WHO diagnostic codes X S Danish Conscription Database (DCD) National Archives Database (NAD) Danish Defence Personnel Organisation Database (DDPOD) Danish Conscription Register (DCR) Danish Conscription Registry Data (DCRD) Population characteristics  No. of observations 728 159 696 585 219 824 404 808 2 049 376  No. of unique individuals 728 159 658 309 219 824 364 665 1 757 242  Examination data,a % 92.0 75.9 99.6 81.5 83.9  Birth year 1929–60 1940–97 1950–88 1973–98 1929–1998  Examination year 1957–84 1987–2011 1995–2005 2006–15 1957–2015  Sex, % males 100 100b 98.9 96.3 99.1  Age at examination, median (IQR) 19 (18–20) 19 (19–20) 19 (19–20) 19 (19–20) 19 (18–20) Examination variables  Personal identification number (CPR-number) X X X X A  Date of birth X X X X A  Date of examination X Xc X X A  Conscription board district X X X S  Municipality X X S  Profession X X X S  Education X X X S  Height (cm) X X X X A  Weight (kg) Xd X S  Cognitive ability X Xe X X Ae  Hair and eye colour X X S  Colour blindness X X S  Eligibility X X X S  WHO diagnostic codes X S X indicates if a variable is present in the individual source of conscription data; A indicates if a variable is present in all four sources; and S indicates if a variable is present in a subset of the four individual sources of conscription data. NA, not available, IQR, interquartile range. a Data on cognitive ability are used as a proxy for presence of examination data. b NAD includes 25 women. c In NAD: a large proportion (14.8%) have missing information on date of examination because they have been exempted from the conscription board examination. d In DCD: weight was not recorded in all conscription districts—see www.regionh.dk/dcd for details on the distribution of weight assessments. e In NAD: cognitive ability was only reported as a categorical variable with the categories 1–5 where category 5 included everyone with a score above the mean. The conscription board examination consists of a questionnaire, an intelligence test, and a health examination and is conducted according to national guidelines. After the examination, the conscription board evaluates whether the conscript is fit for military service, for limited duties or unfit for military service. From 1957, the collected data were recorded in a standardized format on register cards and in 1987, the handling and storage of data were digitized according to the terms and regulations of the Danish Data Protection Agency.5 Today conscription board data are available from the following four sources: the Danish Conscription Database (DCD), the National Archives Database (NAD), the Danish Defence Personnel Organisation Database (DDPOD) and the Danish Conscription Register (DCR). For the period from roughly 1957 through 1977, conscription board data are available in the DCD, which was established for research purposes through manual digitization of conscription board register cards. The Danish Defence Personnel Organisation has digitally recorded key data from all conscription board examinations since 1987. The three other sources are based on these electronic registrations, leaving a gap of almost 10 years (from 1978 through 1986) for which data have not been digitalized. Data collected from 1987 through 2005 have been stored in two separate databases, the NAD and the DDPOD, with a considerable overlap of individuals but differences in the selected variables. The NAD consists of a copy of selected variables for all individuals examined in the period from 1987 through 2011, which has been transferred from the Danish Defence Personnel Organisation to the Danish National Archives. The DDPOD consists of another copy of more detailed variables for individuals examined from 1995 through 2005. This copy has been stored by the Danish Defence Personnel Organisation and predominantly covers men and women who were evaluated as fit for military service or limited duties. Since 2006, a copy of all data collected at the Danish conscription board examinations has been transferred from the conscription authorities to the DCR, which is an official register in the Danish Health Data Authority. As of March 2018 the Danish Health Data Authority has decided to transfer the responsibility of the DCR to the Danish Defence Personnel Organisation, which will handle future updates of the register. The DCRD consists of pooled conscription data from the four sources and provides more than 1.7 million individual observations on men and women born predominantly from 1939 through 1998, who appeared at the conscription board examination from 1957 through 2015. Data collected Coverage Table 1 gives an overview of the main characteristics of the populations covered in the four individual sources of conscription data and combined in the DCRD. The Danish Conscription Database (DCD) The DCD is a database with conscription board examination data for 728 159 men born between 1929 and 1960 (99.8% born between 1939 and 1959) and examined from 1957 through 1984 (99.8% examined between 1958 and 1978). Information on selected variables was digitized and, in case of multiple records on a single individual, the first record containing information on cognitive ability was chosen. For more details on the establishment of the DCD, see Christensen et al. 2015.7 The National Archives Database (NAD) The NAD contains 696 585 observations with data on 658 309 unique individuals of whom only 25 were women. The population was born from 1940 through 1997 (99.8% born between 1969 and 1986) and examined at the conscription board from 1987 through 2011 (83.8% examined between 1988 and 2005). The Danish Defence Personnel Organisation Database (DDPOD) The DDPOD contains conscription data on 219 824 unique individuals (98.9% males) evaluated as fit for military service or limited duties. They were born from 1950 through 1988 (90.7% born between 1976 and 1986) and examined from 1995 through 2005. The Danish Conscription Register (DCR) The DCR is a continuously updated register of conscription data for all men and women examined at the conscription board from 2006 and onwards. The population described in this paper contains conscription data for all men and women examined from 2006 through 2015. The DCR contains 404 808 observations with data on 364.665 unique individuals of whom 96.3% were men. They were born from 1973 through 1998 (97.1% born between 1986 and 1997). Figure 1 presents an overview of the birth cohorts covered by the four sources of conscription board data. The figure clearly illustrates the gap between the DCD and NAD and the overlap between the NAD and DDPOD. The approximate time of conscription board examination is marked by the horizontal line at age 19 (median age at examination). In 2018, the oldest individuals in the DCRD will be 80 years old and the youngest will be 20 years old. The first vertical line on the left represents the introduction of personal identification numbers (CPR-numbers) on 2 April 1968. Figure 1 View largeDownload slide Overview of the follow-up periods for the four sources of conscription board data and the timing of selected national health registers. Sources: DCD: Danish Conscription Database, NAD: National Archives Database, DDPOD: Danish Defence Personal Organisation Database, DCR: Danish Conscription Register. Registers: DRCD: Danish Register of Cause of Death, MBR: Medical Birth Register, DNPR: Danish National Patient Register, NPR: National Prescription Register. CPR-number: Central Person Registration number (personal identification number). Figure 1 View largeDownload slide Overview of the follow-up periods for the four sources of conscription board data and the timing of selected national health registers. Sources: DCD: Danish Conscription Database, NAD: National Archives Database, DDPOD: Danish Defence Personal Organisation Database, DCR: Danish Conscription Register. Registers: DRCD: Danish Register of Cause of Death, MBR: Medical Birth Register, DNPR: Danish National Patient Register, NPR: National Prescription Register. CPR-number: Central Person Registration number (personal identification number). The Danish Conscription Board Data (DCRD) Combining the four samples, the DCRD consists of more than 1.7 million unique individuals examined at the conscription board from 1957 through 2015. The median age at conscription is 19 years (Table 1). Most men (86.0%) are examined between the ages of 18 and 20, but a few (12.8%) have their examination postponed and are examined between the ages of 21 and 26. Men who volunteer before the age of 18 are exempted from appearing before the conscription board but they are still registered, and 0.9% of the men in the DCRD are 17 years old. Moreover, if conscripted, the military service can under special circumstances be postponed until age 32 and a very few (0.3%) are thus re-examined between the ages of 27 and 32. The proportion of individuals for whom examination data (estimated as individuals with data on cognitive ability) are available ranges from around 75% to around 93% and is highest for those born before 1959 and lowest for those born from 1970 through 1979 (Table 1). The proportion with missing examination data mainly represent those exempted as previously described. Given that both those examined and those exempted are registered, the DCRD should cover all Danish men born from 1939 through 1959 and from 1969 through 1997, who have survived until age 18, with the exception of the few who died before being examined. To assess to what degree the DCRD covers the male population residing in Denmark, we divided the DCRD population into 10-year birth cohorts and compared the number of males in the DCRD with the estimated number of 24-year-old males registered in Statistics Denmark (see Table 2). The 24-year-olds were chosen as the reference because the younger age groups (the 18–23 year olds) are characterized by a high level of migration which complicates the comparison and because the number of males is only available for 5-year age groups from age 25 and onwards. Table 2 illustrates how the Danish male population is covered in the DCRD, and coverage is high in all birth cohorts except for those born from 1960 through 1969. This gap represents the gap between the DCD and NAD. Data on these men are available on register cards and are stored in the Danish National Archives. Table 2 Data coverage assessed through comparison of the Danish Conscription Registry Data (DCRD) population and number of males in the corresponding birth cohorts in the Danish population Birth cohort Total 1939–49 1950–59 1960–69 1970–79 1980–89 1990–97 Number of Danish males alive at age 24a 521 963 383 948 387 561 406 013 332 935 279 098 2 311 518 Number of males in DCRD 393 645 333 360 45 596 417 038 308 390 243 042 1 741 071 Proportion (%) of birth cohort covered in DCRD 75.4 86.8 11.8 102.7 92.6 87.1 75.3 Number of males having examination data 364 858 303 862 37 954 313 591 244 707 194 954 1 460 033 Proportion (%) of DCRD having examination datab 92.7 91.2 83.2 75.2 79.3 80.2 83, 9 Birth cohort Total 1939–49 1950–59 1960–69 1970–79 1980–89 1990–97 Number of Danish males alive at age 24a 521 963 383 948 387 561 406 013 332 935 279 098 2 311 518 Number of males in DCRD 393 645 333 360 45 596 417 038 308 390 243 042 1 741 071 Proportion (%) of birth cohort covered in DCRD 75.4 86.8 11.8 102.7 92.6 87.1 75.3 Number of males having examination data 364 858 303 862 37 954 313 591 244 707 194 954 1 460 033 Proportion (%) of DCRD having examination datab 92.7 91.2 83.2 75.2 79.3 80.2 83, 9 a Number of Danish males are derived from Statistics Denmark. The number of males alive at age 26, the age limit for postponement of the conscription board examination, are not available (numbers are grouped in 5-year intervals from age 25 and onwards) and age 24 was thus chosen instead. b Data on cognitive ability are used as a proxy for presence of examination data. Table 2 Data coverage assessed through comparison of the Danish Conscription Registry Data (DCRD) population and number of males in the corresponding birth cohorts in the Danish population Birth cohort Total 1939–49 1950–59 1960–69 1970–79 1980–89 1990–97 Number of Danish males alive at age 24a 521 963 383 948 387 561 406 013 332 935 279 098 2 311 518 Number of males in DCRD 393 645 333 360 45 596 417 038 308 390 243 042 1 741 071 Proportion (%) of birth cohort covered in DCRD 75.4 86.8 11.8 102.7 92.6 87.1 75.3 Number of males having examination data 364 858 303 862 37 954 313 591 244 707 194 954 1 460 033 Proportion (%) of DCRD having examination datab 92.7 91.2 83.2 75.2 79.3 80.2 83, 9 Birth cohort Total 1939–49 1950–59 1960–69 1970–79 1980–89 1990–97 Number of Danish males alive at age 24a 521 963 383 948 387 561 406 013 332 935 279 098 2 311 518 Number of males in DCRD 393 645 333 360 45 596 417 038 308 390 243 042 1 741 071 Proportion (%) of birth cohort covered in DCRD 75.4 86.8 11.8 102.7 92.6 87.1 75.3 Number of males having examination data 364 858 303 862 37 954 313 591 244 707 194 954 1 460 033 Proportion (%) of DCRD having examination datab 92.7 91.2 83.2 75.2 79.3 80.2 83, 9 a Number of Danish males are derived from Statistics Denmark. The number of males alive at age 26, the age limit for postponement of the conscription board examination, are not available (numbers are grouped in 5-year intervals from age 25 and onwards) and age 24 was thus chosen instead. b Data on cognitive ability are used as a proxy for presence of examination data. Data contents The variables included A number of variables are present in all four sources (Table 1). A core variable is the unique personal identification number, the Danish Central Person Register (CPR) number. This 10-digit number combines birth date (in the format DDMMYY), and a sequence number of four digits from which the century of birth and sex of the person can be derived. The sources also include information on the time of conscription board examination. In the DCD, the year and half-year of the examination are registered whereas an examination date, which is registered within 2 weeks of the date of the actual examination, is available in the NAD, DDPOD and DCR. Moreover, information on height and cognitive ability are also present in all four sources. Height (in cm) is measured without shoes and to the nearest cm using a stadiometer. Cognitive ability has been assessed using an intelligence test called Børge Prien’s Prøve (BPP), named after Børge Prien the military psychologist who developed the test. The BPP is a group-administered test consisting of four subtests to be completed within 45 min: (i) letter matrices; (ii) verbal analogies; (iii) number series; and (iv) geometrical figures.4 The BPP consists of 78 items that have remained unchanged from 1957 to the present day, but the test was adapted to a computerized format at the end of 2010. Table 1 also gives an overview of other variables which are present in a subset of the four sources of conscription data. Information on education is available in the DCD, DDPOD and DCR. The men and women examined report their current level of education to the conscription board. From 1957 through 1978, education was coded on a scale from 1 to 9 and reflected a combination of formal schooling and vocational training (for more details see Teasdale and Owen 19868). From 1979 and onwards, education has been coded on a scale from 8 to 13 and reflected number of years of formal schooling. Body weight is only registered in the DCD and DCR, and only the DCR includes information on medical conditions classified according to the international classification of diseases (ICD) system. Six months before the examination, the men and women are required to fill in an electronic form which includes questions on health (e.g. questions on current and previous medical conditions (somatic and psychiatric). A conscription board physician evaluates this information before performing a medical examination at the conscription board examination. Documentation of the individual data sources is available on request. Data quality The conscription board examinations have always been carried out according to national guidelines, by trained medical doctors using standardized methods for measurement of height and weight. As described, the intelligence test (BPP) has also been used without change since 1957 and the total BPP score has been shown to have a substantial correlation(r = 0.82) with the full-scale Wechsler Adult Intelligence Scale.9 As mentioned, the DCD was established through manual digitization of conscription board register cards and, to ensure data quality, 1% of all cards were entered twice by two different people and subsequently compared. The error rates ranged from 0.5% to 1.4%, depending on the variable. Given that around 30% of the DCD men were examined before the introduction of personal identification numbers (CPR-numbers) in 1968, the CPR numbers were traced by matching on name and birth date. This can potentially have led to error in matching, but 88.0% of the DCD men were matched directly (exact same name and birth date, with no difference in spelling of the name) and a smaller percentage of the matched individuals were manually examined, and there was no indication of systematic errors. Data resource use The four sources of conscription data include anthropometric measures and cognitive ability assessed in young adulthood and, due to the unique personal identification number, it is possible to link these data to a large number of Danish health and social registers or cohort studies. The potential of linking the DCRD as well as each individual source of conscription board data to Danish registers is illustrated in Figure 1 by the vertical lines representing the start year of a selection of national health registers. The first line from the left shows the introduction of the personal identification numbers in 1968, which has enabled the linkage of all Danish registries. Until now all four nationwide sources of conscription data have not been used in pooled analysis. DCD has been used as a stand-alone resource in national and international studies of time trends in height10 and BMI.11 It has also been linked to health registers and employed in studies of the impact of cognitive ability on cause-specific mortality and dementia.12,13 In ongoing studies, the DCD is linked to the Copenhagen Aging and Midlife Biobank14 and the Copenhagen School Health Records Register15 in order to study whether changes in overweight over the life course influence later health. The DCR has also been used as a stand-alone resource in studies on trends in BMI16 and has been linked to health registers, e.g. in studies of the impact of infections on cognitive ability in men.17 The DCD, DDPOD and DCR have been used in combined analyses in two completed and several ongoing studies. In a study on the potential bi-directional association between cognitive ability and epilepsy, the authors found that epilepsy (hospital contacts and anti-epileptic treatment) before conscription board examination was associated with lower cognitive ability, and that low cognitive ability at examination was associated with a higher risk of epilepsy later in life.18 The authors thus took advantage of the fact that the youngest individuals in the DCRD could be followed in health registers for information on epilepsy before the examination, and the oldest could be followed after the examination. The conscription board data from the DCD, DDPOD and DCR have also been linked with self-reported data on psychological reactions after returning home from military deployments. In one study the authors found an inverse association between cognitive ability at conscription and post-deployment symptoms of post-traumatic stress disorder,19 and another paper examining the association between cognitive ability and depression is in the pipeline. The possibility to identify family relations for individuals born after 1955, given the unique personal identification number, represents an extremely promising opportunity to study family relations. Table 3 provides an example of such analyses based on the pooled data from the DCD, NAD, DDPOD and DCR that together constitutes the DCRD. It shows that the increasing trend in men’s mean height seemed to level off for birth cohorts born after 1980, and within families the second-born brother seemed to be taller than the first-born during the entire time period under study. Table 3 Mean height and differences in height between first- and second-born brothers across birth cohorts for men in the Danish Conscription Registry Data (DCRD) Birth cohort Total 1939–49 1950–59 1960–69 1970–79 1980–89 1990–97 Number of males in DCRD 393 645 333 360 45 596 417 038 308 390 243 042 1 741 071 Mean height (cm) 175.8 178.0 178.8 180.1 180.2 180.5 178.3 Number (%) with maternal ID 40 946 225 227 39 473 362 672 290 409 242 425 1 200 122 (10.4) (67.6) (86.5) (86.7) (94.2) (99.8) (68.9) Numbers (%) with at least one brother 10 707 74 282 14 565 170 971 164 442 112 048 547 014 (2.7) (22.3) (31.9) (41.0) (53.3) (46.1) (31.4) Mean [SD] difference in height (cm) between second- and first-born brother 0.46 0.43 0.68 0.02 0.18 0.59 0.37 (6.2) (6.5) (6.6) (6.5) (7.6) (7.1) (7.2) Birth cohort Total 1939–49 1950–59 1960–69 1970–79 1980–89 1990–97 Number of males in DCRD 393 645 333 360 45 596 417 038 308 390 243 042 1 741 071 Mean height (cm) 175.8 178.0 178.8 180.1 180.2 180.5 178.3 Number (%) with maternal ID 40 946 225 227 39 473 362 672 290 409 242 425 1 200 122 (10.4) (67.6) (86.5) (86.7) (94.2) (99.8) (68.9) Numbers (%) with at least one brother 10 707 74 282 14 565 170 971 164 442 112 048 547 014 (2.7) (22.3) (31.9) (41.0) (53.3) (46.1) (31.4) Mean [SD] difference in height (cm) between second- and first-born brother 0.46 0.43 0.68 0.02 0.18 0.59 0.37 (6.2) (6.5) (6.6) (6.5) (7.6) (7.1) (7.2) Table 3 Mean height and differences in height between first- and second-born brothers across birth cohorts for men in the Danish Conscription Registry Data (DCRD) Birth cohort Total 1939–49 1950–59 1960–69 1970–79 1980–89 1990–97 Number of males in DCRD 393 645 333 360 45 596 417 038 308 390 243 042 1 741 071 Mean height (cm) 175.8 178.0 178.8 180.1 180.2 180.5 178.3 Number (%) with maternal ID 40 946 225 227 39 473 362 672 290 409 242 425 1 200 122 (10.4) (67.6) (86.5) (86.7) (94.2) (99.8) (68.9) Numbers (%) with at least one brother 10 707 74 282 14 565 170 971 164 442 112 048 547 014 (2.7) (22.3) (31.9) (41.0) (53.3) (46.1) (31.4) Mean [SD] difference in height (cm) between second- and first-born brother 0.46 0.43 0.68 0.02 0.18 0.59 0.37 (6.2) (6.5) (6.6) (6.5) (7.6) (7.1) (7.2) Birth cohort Total 1939–49 1950–59 1960–69 1970–79 1980–89 1990–97 Number of males in DCRD 393 645 333 360 45 596 417 038 308 390 243 042 1 741 071 Mean height (cm) 175.8 178.0 178.8 180.1 180.2 180.5 178.3 Number (%) with maternal ID 40 946 225 227 39 473 362 672 290 409 242 425 1 200 122 (10.4) (67.6) (86.5) (86.7) (94.2) (99.8) (68.9) Numbers (%) with at least one brother 10 707 74 282 14 565 170 971 164 442 112 048 547 014 (2.7) (22.3) (31.9) (41.0) (53.3) (46.1) (31.4) Mean [SD] difference in height (cm) between second- and first-born brother 0.46 0.43 0.68 0.02 0.18 0.59 0.37 (6.2) (6.5) (6.6) (6.5) (7.6) (7.1) (7.2) Strengths and weaknesses The DCRD covers a considerable proportion of Danish men born from 1939 through 1959 and from 1969 onwards. Whereas conscription is mandatory for men, which ensures a high level of generalizability of the male DCRD population, the few women in the DCRD represent a selected group who have volunteered for military service. Only relatively few variables of interest for health research are included. However, the information has been collected by trained medical doctors using standardized methods. The coverage or coding of some of the relevant variables, such as weight and cognitive ability, varies across the four sources of conscription data. However, in combination with data from Danish registers, measures of young adult physical and mental characteristics can be used as exposures, covariables and outcomes in epidemiological studies of health across the life course. The DCD differs from the other three sources of data given that it was established through manual digitization and only one record per person was digitized. Furthermore, the DDPOD only contains one record per person and the population is for the most part restricted to those who were evaluated as fit for military service or limited duties. Consequently, the individuals having the poorest scores on the individual variables have been excluded. Even though the DDPOD population is also present in the NAD, the DDPOD is included because of better information on cognitive ability compared with the NAD. Data resource access Access to the Danish Conscription Register Data including personal identification numbers can only be obtained with approval from the Danish Data Protection Agency [www.datatilsynet.dk]. Researchers can contact the individual sources of conscription data with their project description and approval as follows. The DCD can be contacted through the homepage [www.regionh.dk/dcd]. Data from the NAD and DCR can be applied for by filling out a form found on the websites of the National Danish Archives [https://www.sa.dk/brug-arkivet/bestil/digitale-arkivalier] or the Danish Health Data Authority [http://sundhedsdatastyrelsen.dk/da/forskerservice/ansoegningsskema/ansoegningsskema-fsk]. As the Danish Health Data Authority has decided to transfer the responsibility of the DCR to the Danish Defence Personnel Organisation, requests for access to DCR will in the future be managed by the Danish Defence Personnel Organisation. Data from the DDPOD can be obtained by submission of an application to the Danish Defence Personnel Organisation. Profile in a nutshell The DCRD was established to take advantage of the unique data on physical and mental characteristics of all young Danish men collected at the Danish conscription board examinations. The DCRD consists of conscription data on 1.7 million Danish citizens (99.1% males) examined at the Danish conscription board from 1957 through 2015 and covers around 75% of Danish men born from 1939 through 1997. By law all Danish men, with the exemption of those having a serious medical condition (e.g. epilepsy, type I diabetes or intellectual disability), are required to appear before the conscription board between ages of 18 to 26. From 1962, women volunteers have been accepted in the military but still remain a minority (0.9% in DCRD). The conscription board examination is carried out according to standardized national guidelines and consists of a questionnaire, an intelligence test and a physical examination. The data collected have been recorded in a standardized format, with few changes from 1957 up to today. The main categories of data collected are personal identification number, sex, birth date, date of conscription examination, height, education and cognitive ability. Moreover, the data can be linked to all Danish registry data using the personal identification number. Access to DCRD including personal identification numbers requires approval from the Danish Data Inspection Agency. For each source of data, access can be applied for as follows: the DCD—contact the steering group through [www.region.dk/dcd]; the NAD—contact the National Danish Archives [www.sa.dk]; DDPOD—contact Danish Defence Personnel Organisation [www.forpers.dk]; DCR—contact the Danish Health Data Authority [www.sundhedsdatastyrelsen.dk]. Funding This work was mainly supported by the Danish Medical Research Council (grant numbers 09–063599 and 09–069151), the Velux Foundation (grant number 95–103-11419), the Jascha Foundation (grant number 6200) and Doctor Sofus Carl Emil Friis and Olga Doris Friis grant. Conflict of interest: The authors declare no conflict of interest. Author Contributions M.O. conceived the study idea. G.T.C., M.O. and S.S. made the analyses and wrote the initial draft. All authors contributed to critical revision of the paper and agreed to be accountable for all aspects of the work. References 1 Lynch J , Davey Smith G. A life course approach to chronic disease epidemiology . Ann Rev Public Health 2005 ; 26 : 1 – 35 . Google Scholar CrossRef Search ADS 2 Kuh D , Ben-Shlomo Y , Lynch J , Hallqvist J , Power C. Life course epidemiology . J Epidemiol Community Health 2003 ; 57 : 778 – 83 . Google Scholar CrossRef Search ADS PubMed 3 Cole M , Cole SR , Lightfood C. The Development of Children , 5th edn . New York, NY : Worth Publishers , 2005 . 4 Teasdale TW. The Danish draft board’s intelligence test, Borge Priens Prove: psychometric properties and research applications through 50 years . Scand J Psychol 2009 ; 50 : 633 – 38 . Google Scholar CrossRef Search ADS PubMed 5 Green A. The Danish Conscription Registry: a resource for epidemiological research . 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Published by Oxford University Press on behalf of the International Epidemiological Association This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

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International Journal of EpidemiologyOxford University Press

Published: Apr 11, 2018

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