Incidence rates of cancer vary over time;1 this is called a “period effect”. Incidence rates of cancer also tend to increase with age;1 this is called an “age effect”. Another time variable that is associated with the incidence of cancer is year of birth of the patient — the “cohort effect”. Many authors have used real data to emphasise the importance of cohort effects for assessing trends in incidence.2,3 Here, I present a simple, hypothetical example to illustrate the cohort effect.
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- 1. Australian Institute of Health and Welfare. Cancer in Australia 2010: an overview. Canberra: AIHW, 2010. (AIHW Cat. No. 56; Cancer Series No. 60.)
- 2. Gardner MJ, Osmond C. Interpretation of disease time trends: is cancer on the increase? A simple cohort technique and its relationship to more advanced models. J Epidemiol Community Health 1983; 37: 274-278.
- 3. Holford TR. Understanding the effects of age, period, and cohort on incidence and mortality rates. Annu Rev Public Health 1991; 12: 425-457.
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