Which measure is most appropriate when follow-up times vary across participants?

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

Answer: (B)

When follow-up times vary across participants, you want a measure that accounts for the actual time each person is at risk. Incidence rate (incidence density) does this by using person-time in the denominator. You tally the time each participant contributes while they are at risk and divide the number of new cases by the total person-time, often reporting per 1,000 or 100,000 person-years. This approach correctly gives more weight to longer follow-up and less to shorter follow-up, so the measure remains valid even when follow-up lengths differ.

Cumulative incidence, by contrast, assumes a fixed, complete follow-up for everyone and can be biased when follow-up is incomplete or unequal. Prevalence reflects the proportion of people with the disease at a specific point (or period) in time and does not describe the rate at which new cases occur. The odds ratio is a ratio of odds used to measure association between exposure and outcome, not a time-based rate.

So the most appropriate measure in the setting of varying follow-up times is incidence rate.