Which statement correctly defines incidence rate in comparison to prevalence?

• A. Incidence rate equals new cases divided by person-time at risk.
• B. Prevalence equals new cases divided by person-time at risk.
• C. Incidence rate measures the burden of disease at a point in time.
• D. Prevalence is calculated as the number of new cases divided by the population at risk.

Answer: (A)

Incidence rate measures how quickly new cases arise over time. It uses the number of new (incident) cases in the numerator and the total amount of time each person was at risk in the denominator, summed across all people observed. This person-time approach accounts for people entering and leaving the at-risk population and for varying follow-up times, so the rate reflects the speed of new disease development, not a single snapshot.

In contrast, prevalence reflects how widespread the disease is at a specific moment (or over a period) and is calculated as existing cases divided by the population at that time. It measures burden, not the speed of new cases. So the statement that incidence rate equals new cases divided by person-time at risk is the correct definition, while descriptions tying incidence to a point-in-time burden or tying prevalence to new cases divided by person-time are not correct.