Which measure is most appropriate for assessing the risk of developing a disease over a defined period?

• A. Incidence measures existing cases.
• B. Prevalence measures new cases.
• C. Incidence and prevalence are the same.
• D. Incidence measures new cases; prevalence measures existing cases.

Answer: (D)

When you want to know the chance that someone will develop a disease during a defined time frame, you need a measure that follows people who are initially disease-free and counts how many of them actually develop the disease in that period. This is incidence. For a fixed follow-up period, cumulative incidence—the proportion of at-risk individuals who become cases during the interval—gives the risk of developing the disease over that time. Prevalence, in contrast, is the proportion of people who have the disease at a given time and includes those with long-standing disease as well as new cases; it reflects how widespread the disease is at that moment, not the risk of developing it during the interval. So, incidence measures new cases (the risk over time), while prevalence measures existing cases.