Age effects account for the biological effects of age on incidence, stripped of the effects due to changes in exposure, and other changes such as changes in reporting rates. Period effects account for influences affecting the incidence in all age groups simultaneously, such as a change in case definition, or case ascertainment and reporting (surveillance). Cohort effects account for influences affecting the incidence in a specified birth cohort equally throughout life.

Because in CJD the incubation time is long and variable among persons, changes in exposure to unkown environmental factors are better described as cohort effects than as period effects. This comes from the fact that persons born in the same generation are most likely exposed to the same risks: they have contemporary and similar living habits (e.g., eating habits, working conditions). By contrast, if the incubation time was fixed, changes in exposure among cohorts would appear as period effects: subjects exposed in 1940 would appear as cases at the same date (e.g., in 1960 if the incubation was 20 years for everyone).