Two common measures of incidence are

• A. Cumulative incidence and incidence rate
• B. Point prevalence and period prevalence
• C. Relative and absolute incidence
• D. Crude and adjusted incidence

Answer: (A)

Incidence is about new cases appearing over time, so the main idea is how we quantify how quickly those new cases arise in a population. The two common measures are cumulative incidence and incidence rate.

Cumulative incidence represents the probability that a person at risk develops the disease during a defined period. It is calculated as the number of new cases during the period divided by the number of individuals at risk at the start. It’s a proportion, giving a sense of risk over the specified time.

Incidence rate, or incidence density, describes how fast new cases occur, taking into account the actual time each person is observed. It is the number of new cases divided by the total person-time at risk, expressed as cases per person-time (for example, per 1,000 person-years). This measure is useful when follow-up times vary among individuals or when the observation period isn’t uniform.

The other options mix prevalence measures or nonstandard terminology. Point and period prevalence quantify how many people have the disease at a moment or over a period, not how many develop it. Relative and absolute incidence aren’t standard paired measures of incidence, and crude versus adjusted incidence refers to standardization or adjustment methods, not two primary incidence measures themselves.

So, focusing on the core idea of incidence as new cases over time, cumulative incidence and incidence rate are the two most commonly used measures.