Incidence density is another term for which measure?

• A. Cumulative incidence
• B. Prevalence
• C. Risk ratio
• D. Incidence rate

Answer: (D)

Incidence density measures the speed at which new cases appear in a population, using the amount of person-time each person contributes while at risk. It is calculated as the number of new cases divided by the total time at risk across all individuals, commonly expressed as cases per person-time (for example, per 1,000 person-years). This approach matters because people are observed for different lengths of time, due to staggered entry, loss to follow-up, or varying follow-up durations, and using person-time accounts for that variability.

This is the same concept as the incidence rate, so incidence density is another way to denote incidence rate. It differs from cumulative incidence, which is a proportion of a fixed cohort that develops disease over a specified period and does not explicitly account for how long each person was observed. It also differs from prevalence, which reflects the total number of existing cases at a point or period, not the rate at which new cases occur. For example, if 3 new cases arise over 150 person-years of observation, the incidence density (incidence rate) would be 3/150 = 0.02 cases per person-year (or 20 per 1,000 person-years).