LESSON 4

MEASURES OF DISEASE FREQUENCY

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?

Prevalence

Point Prevalence

Example

Period Prevalence

Mortality