What is incidence rate (incidence density) and how is it calculated?

• A. Incidence rate is the proportion of people who develop disease during follow-up.
• B. Incidence rate is the number of new cases divided by the total population.
• C. Incidence rate (incidence density) is the number of new cases per person-time of observation; calculated as (new cases) / (sum of person-time).
• D. Incidence rate is the ratio of new cases to existing cases.

Answer: (C)

Incidence rate, or incidence density, measures how fast new disease cases occur in a population by accounting for the exact amount of time people are at risk. It uses person-time in the denominator, because people are observed for different lengths of time and some may be lost to follow-up.

You calculate it by summing the amount of time each person is at risk and under observation, then dividing the number of new cases by that total person-time. It’s usually expressed as cases per person-time (for example, per 1,000 person-years). For instance, if you observe a total of 2,000 person-years and observe 7 new cases, the incidence rate is 7 divided by 2,000, or 0.0035 per person-year (which is 3.5 per 1,000 person-years).

The other descriptions describe different measures. A describes the proportion of people who develop disease during follow-up (cumulative incidence or risk), which does not incorporate how long each person was observed. B and the idea of dividing by the total population also give a proportion, not a rate that accounts for time. D refers to a relationship between new and existing cases, not the speed of new cases over time.