Cumulative Incidence (“Risk”)

In EKS “Incidence Proportion”



Problems with Simple Cumulative Incidence


Loss to follow-up

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:



Calculation of a Rate




Determining Person Time Information


Only need to understand 2 methods for this class:

·          Summing up individual disease-free time

·          Estimating PT for one year


Risk versus Rate


Incidence can be measured as either risk or rate.  Which of these types to use is an important choice when planning an epidemiologic study.


          0 < Risk < 1


0 < Rate <


When planning an epidemiologic study, which measure to use, risk or rate?




Point Prevalence






Period Prevalence