The Effect of a Physical Activity Intervention on Bias in Self-Reported
DANIEL R. TABER,
, JUNE STEVENS,
, DAVID M. MURRAY,
, JOHN P. ELDER,
LARRY S. WEBBER,
, JARED B. JOBE,
, AND LESLIE A. LYTLE,
PURPOSE: A positive outcome in self-reported behavior could be detected erroneously if an intervention
caused over-reporting of the targeted behavior. Data collected from a multi-site randomized trial were
examined to determine if adolescent girls who received a physical activity intervention over-reported their
activity more than girls who received no intervention.
METHODS: Activity was measured using accelerometers and self-reports (3-Day Physical Activity
Recall, 3DPAR) in cross-sectional samples preintervention (6th grade, n Z 1,464) and post-intervention
(8th grade, n Z 3,114). Log-transformed accelerometer minutes were regressed on 3DPAR blocks, treat-
ment group, and their interaction, while adjusting for race, body mass index, and timing of data collection.
RESULTS: Preintervention, the association between measures did not differ between groups, but post-
intervention 3DPAR blocks were associated with fewer log-accelerometer minutes of moderate-vigorous
physical activity (MVPA) in intervention girls than in control girls (p Z 0.002). The group difference
was primarily in the upper 15% of the 3DPAR distribution, where control girls had O1.7 more accelerom-
eter minutes of MVPA than intervention girls who reported identical activity levels. Group differences in
this subsample were 8.5%–16.2% of the mean activity levels; the intervention was powered to detect
a difference of 10%.
CONCLUSION: Self-report measures should be interpreted with caution when used to evaluate a phys-
ical activity intervention.
Ann Epidemiol 2009;19:316–322. Ó 2009 Elsevier Inc. All rights reserved.
: Bias (epidemiology), Intervention Studies, Adolescent, Exercise, Questionnaires, Social
Self-report measures of physical activity are a popular means
of evaluating physical activity interventions because they
are more feasible and less expensive than objective measures
of activity (e.g., accelerometers). Among the 76 physical
activity intervention studies in a recent review by Salmon
et al. (1), 51 relied exclusively on self-report measures.
Some experts have argued, however, that self-report
measures are insufﬁcient for intervention studies because
of their potential for misclassiﬁcation (2, 3). Jacobs et al.
(4) reviewed 10 of the most commonly used activity self-
report questionnaires and found that all were poorly corre-
lated with accelerometer-measured activity. Sirard and
Pate (5) reviewed common self-report measures of activity
in children and adolescents and found a wide range of corre-
lations with objective measures of activity (r Z À0.10 to
Observational studies of self-reported physical activity
often acknowledge such misclassiﬁcation as a limitation,
but argue that it is nondifferential and assume that effect
estimates are conservative. The same logic cannot neces-
sarily be applied to intervention studies because differential
misclassiﬁcation could result from social desirability bias
induced by the intervention. Social desirability is ‘‘the
defensive tendency of individuals to respond in a manner
that is consistent with social norms or beliefs’’ (6). Compar-
isons of self-reported to objective measures of physical
activity have indicated that social desirability is associated
with over-reporting among females (7, 8). Interventions,
in their effort to encourage individuals to change behavior,
may promote social desirability and inadvertently increase
over-reporting of that behavior.
Such differential misreporting has been noted in behav-
ioral intervention studies aimed at changing diet behaviors.
Espeland et al. (9) reported that participants who were
randomized to receive a sodium reduction and weight loss
intervention underreported their sodium intake more than
participants randomized to other groups. Harnack et al.
(10) similarly found that girls randomized to receive an
From the Departments of Epidemiology (D.R.T., J.S.) and Nutrition
(J.S.), University of North Carolina, Chapel Hill; Division of Epidemi-
ology, College of Public Health, The Ohio State University, Columbus
(D.M.N.); Division of Health Promotion and Behavioral Sciences, San
Diego State University, San Diego, CA (J.P.E.); Department of Biostatis-
tics, School of Public Health and Tropical Medicine, Tulane University,
New Orleans, LA (L.S.W.); the Division of Prevention and Population
Sciences, National Heart, Lung, and Blood Institute, Bethesda, MD
(J.B.J.); and the Division of Epidemiology and Community Health, Univer-
sity of Minnesota, Minneapolis (L.A.L.).
Address correspondence to: Daniel Taber, Department of Epidemiology,
CB#7435, University of North Carolina, Chapel Hill NC 27599. Tel.:
(919) 966-0117. Fax: (919) 966-7215. E-mail: email@example.com.
Received July 28, 2008; accepted January 8, 2009.
Ó 2009 Elsevier Inc. All rights reserved. 1047-2797/09/$–see front matter
360 Park Avenue South, New York, NY 10010 doi:10.1016/j.annepidem.2009.01.001