EVALUATION OF BIRTH COHORT PATTERNS IN POPULATION DISEASE RATES

Interpretation of trends in disease rates using conventional age-perio d-cohort analyses is made difficult by the lack of a unique set of par ameters specifying any given model. Because of difficulties inherent i n age-period-cohort models, neither the magnitude nor the direction of a linear trend in birth cohort effects or calendar period effects can be determined unambiguously. This leads to considerable uncertainty i n making inferences regarding disease etiology based on birth cohort o r calendar period trends. In this paper, the authors demonstrate that changes in the direction or magnitude of long term trends can be ident ified unequivocally in age-period-cohort analyses, and they provide pa rametric methods for evaluating such changes in trend within the usual Poisson regression framework. Such changes can have important implica tions for disease etiology. This is demonstrated in applications of th e proposed methods to the investigation of birth cohort trends in fema le breast cancer mortality rates obtained from the National Center for Health Statistics for the United States (1970-1989) and from the Worl d Health Organization for Japan (1955-1979).