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Visual Inspection of Age-Specific Incidence Rates : Don't Forget the Scaling!

Visual Inspection of Age-Specific Incidence Rates : Don't Forget the Scaling! In a recent article, Hemminki et al1 reported time trends and familial risks in squamous cell carcinoma (SCC) of the skinbased on the Swedish Family-Cancer Database. In particular, they tried to address this question with reference to sun-exposed and covered sites of thebody. Figure 3 of their article displays the age-specific incidence ratesof invasive SCC of the skin on sun-exposed and covered body parts. They definedthat sun-exposed sites among males "included the head, neck, and arms, and for women, also legs. All other sites were considered nonexposed." From Figure3 they conclude, "the slopes (incidence/age) are quite different for sun-exposed and covered sites." We think that this conclusion is wrong. They displayedincidence rates by age group on a linear scale, which simply masks the slope of the age-specific incidence rate of the covered skin sites. If the authorswould have used a logarithmic scale, they would have found that the slopes of the age-specific incidence rates of sun-exposed and covered skin sitesare very similar. It is well known that using linear scales hampers the visual assessment of proportional rates of change. The use of a logarithmic scalefor the y-axis will result in a straight line when the rate of change is constant. Parallel lines on a logarithmic scale for 2 or more groups indicate the samerate of change.2 We ran the same analyses as Hemminki et al1 based on the data of the population-based cancer registry of the Federal State ofSaarland, Gemany, for the calendar period 1995 through 1999.3Table 1 displays the overall and site-specific incidence rates of first invasive SCC in the Federal State of Saarland formen and women. We used the same definition of sun-exposed and covered bodysites as Hemminki et al (sun-exposed sites for men: head, neck, and arms;and for women: head, neck, arms, and legs; all others sites were considerednonexposed). View LargeDownload First Invasive Squamous Cell Carcinoma of the Skin, Saarland, Germany, 1995-1999 The incidence rates for covered sites are considerably lower than for sun-exposed sites so that the graphical presentation on a linear scale masksthe slope of the age-specific incidence rate of covered sites. Figure 1 demonstrates the effect of choosing a linear and logarithmic scale to display the age-specific rates. View LargeDownload Age-specific incidence rates of invasive squamous cell carcinomaof the skin on sun-exposed and covered body sites (A, linear scale; B, logarithmicscale), Saarland, Germany, 1995-1999. We conclude that the slopes (incidence/age) are very similar for sun-exposedand covered sites. The authors have no relevant financial interest in this letter. References 1. Hemminki KZhang HCzene K Time trends and familial risks in squamous cell carcinoma of the skin Arch Dermatol. 2003;139885- 889PubMedGoogle Scholar 2. Devesa SSDonaldson JFears T Graphical presentation of trends in rates Am J Epidemiol. 1995;141300- 304PubMedGoogle ScholarCrossref 3. Stang AStegmaier CJöckel KH Nonmelanoma skin cancer in the Federal State of Saarland, Germany, 1995-1999 Br J Cancer. 2003;891205- 1209PubMedGoogle ScholarCrossref http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Archives of Dermatology American Medical Association

Visual Inspection of Age-Specific Incidence Rates : Don't Forget the Scaling!

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Publisher
American Medical Association
Copyright
Copyright © 2004 American Medical Association. All Rights Reserved.
ISSN
0003-987X
eISSN
1538-3652
DOI
10.1001/archderm.140.3.361-b
Publisher site
See Article on Publisher Site

Abstract

In a recent article, Hemminki et al1 reported time trends and familial risks in squamous cell carcinoma (SCC) of the skinbased on the Swedish Family-Cancer Database. In particular, they tried to address this question with reference to sun-exposed and covered sites of thebody. Figure 3 of their article displays the age-specific incidence ratesof invasive SCC of the skin on sun-exposed and covered body parts. They definedthat sun-exposed sites among males "included the head, neck, and arms, and for women, also legs. All other sites were considered nonexposed." From Figure3 they conclude, "the slopes (incidence/age) are quite different for sun-exposed and covered sites." We think that this conclusion is wrong. They displayedincidence rates by age group on a linear scale, which simply masks the slope of the age-specific incidence rate of the covered skin sites. If the authorswould have used a logarithmic scale, they would have found that the slopes of the age-specific incidence rates of sun-exposed and covered skin sitesare very similar. It is well known that using linear scales hampers the visual assessment of proportional rates of change. The use of a logarithmic scalefor the y-axis will result in a straight line when the rate of change is constant. Parallel lines on a logarithmic scale for 2 or more groups indicate the samerate of change.2 We ran the same analyses as Hemminki et al1 based on the data of the population-based cancer registry of the Federal State ofSaarland, Gemany, for the calendar period 1995 through 1999.3Table 1 displays the overall and site-specific incidence rates of first invasive SCC in the Federal State of Saarland formen and women. We used the same definition of sun-exposed and covered bodysites as Hemminki et al (sun-exposed sites for men: head, neck, and arms;and for women: head, neck, arms, and legs; all others sites were considerednonexposed). View LargeDownload First Invasive Squamous Cell Carcinoma of the Skin, Saarland, Germany, 1995-1999 The incidence rates for covered sites are considerably lower than for sun-exposed sites so that the graphical presentation on a linear scale masksthe slope of the age-specific incidence rate of covered sites. Figure 1 demonstrates the effect of choosing a linear and logarithmic scale to display the age-specific rates. View LargeDownload Age-specific incidence rates of invasive squamous cell carcinomaof the skin on sun-exposed and covered body sites (A, linear scale; B, logarithmicscale), Saarland, Germany, 1995-1999. We conclude that the slopes (incidence/age) are very similar for sun-exposedand covered sites. The authors have no relevant financial interest in this letter. References 1. Hemminki KZhang HCzene K Time trends and familial risks in squamous cell carcinoma of the skin Arch Dermatol. 2003;139885- 889PubMedGoogle Scholar 2. Devesa SSDonaldson JFears T Graphical presentation of trends in rates Am J Epidemiol. 1995;141300- 304PubMedGoogle ScholarCrossref 3. Stang AStegmaier CJöckel KH Nonmelanoma skin cancer in the Federal State of Saarland, Germany, 1995-1999 Br J Cancer. 2003;891205- 1209PubMedGoogle ScholarCrossref

Journal

Archives of DermatologyAmerican Medical Association

Published: Mar 1, 2004

Keywords: inspection

References