Which statement correctly defines incidence?

• A. New cases during a period divided by population at risk
• B. All existing cases at a point in time divided by total population
• C. Cumulative cases over lifetime divided by population
• D. Cases per 1000 people per year only for chronic diseases

Answer: (A)

Incidence measures the risk that a person who is disease-free at the start will develop the disease during a defined time period. It is calculated as the number of new cases during that period divided by the population at risk, often expressed per 1,000 or per 100,000 per year. This matches the statement because it uses new cases over a period relative to the population at risk. The alternative describing all existing cases at a single point in time is prevalence, not incidence. The idea of lifetime or cumulative cases divided by population points toward lifetime risk or cumulative incidence, which has a different interpretation and usually follows people over time with fixed follow-up. And incidence applies to any disease, not only chronic ones.