What is the incidence rate formula?

• A. (# new cases of disease occurring in population during specified time / Total population at risk of developing disease during that time) × 1000
• B. (Total population at risk during that time / # new cases) × 1000
• C. (# new cases / total population) × 100
• D. (# deaths / total population) × 1000

Answer: (A)

Incidence rate measures how quickly new cases appear in a population over a period. It is normally calculated as the number of new cases divided by the amount of time people were at risk (person-time), often scaled to a conventional unit like per 1,000 or per 100,000 person-time.

Among the options, the one that uses the number of new cases during the period divided by the population at risk during that period, then scaled by 1,000, best matches this concept in a practical form. It reflects the idea of new cases relative to those who could develop disease during the time window.

The other expressions don’t fit as well: swapping numerator and denominator changes the meaning entirely, using the total population instead of those at risk ignores how many were actually at risk during the period, and using deaths rather than new cases measures mortality, not incidence.