In epidemiology, what measure describes the risk of disease in a population over a specified period?

• A. Mortality rate
• B. Prevalence
• C. Incidence rate
• D. Relative risk

Answer: (C)

When you want to describe how quickly new cases appear in a population during a defined time window, the incidence rate is the appropriate measure. It accounts for the amount of time each person is at risk, summing up the actual person-time contributed and expressing new cases per unit of that time (for example, per 1,000 person-years). This time-based denominator makes incidence rate robust when people enter or exit the population, are lost to follow-up, or have varying observation periods.

Mortality rate focuses on deaths, not new disease cases. Prevalence measures how many people currently have the disease at a point or over a period, a snapshot of existing cases rather than the rate of new ones. Relative risk compares the likelihood of disease between two groups, assessing association rather than describing the overall rate of new cases in the population over time.

Example: if 12 new cases occur over 600 person-years of observation, the incidence rate is 12 divided by 600, or 0.02 cases per person-year (20 per 1,000 person-years). This illustrates the speed of disease emergence with time taken into account.