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American Journal of Epidemiology Vol. 140, No. 7: 649-657
Copyright © 1994 by The Johns Hopkins University School of Hygiene and Public Health

research-article

Calendar Effects in the Analysis of Seasonal Data

S. D. Walter

Department of Clinical Epidemiology and Biostatistics, McMaster University Hamilton, Ontario L8N 3Z5, Canada

the Department of Clinical Epidemiology and Biostatistics, McMaster University, Hamilton, Ontario L8N 3Z5, Canada. (Reprint requests to Dr. Walter at this address.)

A variety of statistical methods exist for analyzing seasonal patterns in epidemiologic data. As a simplification in the calculations, these methods often do not explicitly take into account certain calendar effects, such as the variation in month length, the irregular number of weekend days in each month, and the occurrence of holidays. This paper evaluates the bias caused by failing to recognize these effects. It is found that with the sample sizes commonly encountered in this type of analysis of epidemiologic data, calendar effects have a high probability of producing a spuriously significant seasonal effect, the amplitude of which may be of the same order of magnitude as the true un derlying seasonal trend. Therefore, it is recommended that calendar effects be routinely taken into account, and some methods for doing so are proposed.

data interpretation; statistical; epidemiologic methods; morbidity; seasons; vital statistics


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