MEASURES OF DISEASE FREQUENCY
Different periods of time of follow-up
Dynamic population (Open population in EKS)
Fixed cohort (Closed Cohort in EKS)
Example with 12 initially disease-free subjects followed over a 5-year period from 1990 to 1995.
X denotes time subject diagnosed with the disease and a circle (O) denotes time of death that could be due to the disease (circle with an X inside) or due to another cause (circle without an X). “Shifting” the cohort:
0 < Risk < 1
0 < Rate <
When planning an epidemiologic study, which measure to use, risk or rate?