Which statement correctly distinguishes risk from incidence rate?

• A. Risk is a probability over a fixed period; incidence rate uses person-time to account for variable follow-up.
• B. Risk uses person-time to account for varying follow-up times.
• C. Incidence rate is the probability over a fixed period.
• D. Risk and incidence rate are the same concept.

Answer: (A)

The key idea here is how each measure treats time. Risk (cumulative incidence) is the probability that an individual will develop the disease during a defined, fixed period. It’s typically calculated as the number of new cases during that period divided by the number at risk at the start, assuming follow-up is complete for that interval.

Incidence rate (incidence density) is a rate, not a simple probability, and it uses person-time to account for how long each person is actually observed. It’s the number of new cases divided by the total amount of person-time contributed by all people in the study, so it handles varying follow-up times, losses to follow-up, and staggered entry.

So the statement that correctly distinguishes them is that risk is a probability over a fixed period, while incidence rate uses person-time to account for variable follow-up. The other descriptions mix up the definitions or imply they are the same concept.