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To the Editor.—
The article in the Archives (112:1397-1399,1976) by Drs Carter and Jegasothy on alopecia areata in patients with Down's syndrome was interesting and useful. The authors reported alopecia areata in 19 of 214 patients, and the authors reported the incidence of the disease to be 17.4% in females and 3.1% in males.I am writing to call attention to the difference between the terms "incidence" and "prevalence." Incidence is the number of new cases per year per a fixed unit of population. For example, the incidence of melanoma might be reported as 6.5 cases per 100,000 population for the year 1973 in Alameda, Calif. In contrast, prevalence refers to the number of patients affected with a particular disease in a unit population at one specific point in time. I think the authors of this article used incidence where prevalence would have been more accurate. This is a common
Tkach JR. Incidence vs Prevalence. Arch Dermatol. 1977;113(12):1731. doi:10.1001/archderm.1977.01640120099031
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