Which statement correctly relates cumulative incidence and incidence proportion?

• A. They measure different concepts and are unrelated.
• B. They measure incidence rate.
• C. They represent the proportion of the initially disease-free population that develops disease during the period.
• D. They measure prevalence.

Answer: (C)

Cumulative incidence, also called incidence proportion, captures the risk of developing the disease over a defined period in a population that is free of the disease at the start. It is computed as the number of new cases during the period divided by the number of people who were disease-free at baseline. Because both the numerator and denominator refer to a fixed group over a specified time, it is expressed as a proportion (a fraction between 0 and 1) and can be read as the proportion or percent of the initial disease-free population that becomes diseased during that period.

This differs from incidence rate, which uses person-time to account for varying follow-up and reflects how fast new cases occur. It also differs from prevalence, which counts existing cases at a point in time (or over a period), not new cases. Therefore, the statement that cumulative incidence represents the proportion of the initially disease-free population that develops disease during the period is the correct relationship.