What are the two methods of measuring incidence?

• A. Cross-sectional survey and case-control study
• B. Cohort followed over time and incidence density in a dynamic population
• C. Prevalence study and attack rate
• D. Point prevalence and period prevalence

Answer: (B)

Incidence is about new disease occurrences over time. The two standard ways to measure it are: following a group that starts free of the disease over time and recording who develops the disease, which gives cumulative incidence (risk) over that period; and, in populations where people enter or leave the population, computing incidence density (incidence rate) as the number of new cases divided by the total person-time at risk. The first method provides a straightforward risk over a defined period in a fixed cohort, while the second accounts for changing population membership by using person-time as the denominator. Cross-sectional or case-control designs don’t track new cases over time to produce incidence, and prevalence measures (point or period prevalence) capture existing cases, not new ones; attack rate is a special cumulative-incidence measure used in outbreaks but not a general method for measuring incidence in a population.