Define incidence.

• A. Number of new cases divided by the total population
• B. Number of new cases divided by the number of people at risk
• C. Number of existing cases divided by the population
• D. Proportion of population with disease at a given time

Answer: (B)

Incidence measures how fast new cases of a disease are appearing in a population that is at risk during a specified time period. The numerator is the number of new cases that develop, and the denominator is the number of people who are at risk of developing the disease (those who do not already have it and are susceptible).

Using the total population as the denominator includes people who aren’t at risk, which distorts the rate. In contrast, incidence focuses on new occurrences among those who could become cases. Prevalence, on the other hand, uses existing cases divided by the population, reflecting how widespread the disease is at a point or over a period, rather than how quickly new cases arise.