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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