Incidence is best described as

• A. Number of NEW cases that develop over a specific period of time
• B. The total number of existing cases at a point in time
• C. The rate of mortality among cases
• D. The rate of exposure in population

Answer: (A)

Incidence refers to new disease events occurring in a population over a defined period, reflecting how likely people who are at risk are to develop the disease. It focuses on new cases that arise during the time frame, excluding those who already had the condition at the start. For example, if you follow 1,000 people who do not have the disease and 50 develop it over one year, the incidence is 50 per 1,000 (5% for that period). Incidence can be expressed as a cumulative measure (a risk over the period) or as an incidence rate using person-time, but both concepts center on new occurrences. This differs from prevalence, which counts all existing cases at a point in time regardless of when they began, and from mortality rate among cases or rate of exposure, which describe other aspects of disease or population dynamics rather than new disease development.