Incidence Rate: Correct Characteristics

Incidence rate measures the frequency of new cases of a disease occurring in a defined population during a specified period of time. The correct answer is 2, 3 and 4.

Evaluation of Each Statement

• Statement 1: "It is a sum total of new and old cases." — Incorrect. Incidence strictly counts only new cases arising during the observation period. Old (pre-existing) cases are excluded from incidence; a measure that includes both new and old cases is prevalence, not incidence. This is a common point of confusion, but it is a defining feature that distinguishes incidence from prevalence.
• Statement 2: "It refers to a specified period of time." — Correct. Incidence rate is always calculated over a defined time interval (e.g., per day, month, or year), since it reflects the rate at which new cases develop.
• Statement 3: "It refers to a defined population." — Correct. Incidence is expressed as new cases divided by the population at risk during that period, so it is inherently population-specific (e.g., per 1,000 population).
• Statement 4: "It must include the unit of time in final expression." — Correct. Because incidence reflects the rate of occurrence of new events, its final expression must state the time dimension (e.g., "cases per 1,000 population per year"), which is what makes it a true "rate."

Why the Other Options Are Wrong

• 1, 3 and 4: Incorrectly includes statement 1 (which wrongly adds old cases to the definition of incidence) and omits statement 2 (time period), which is essential to defining incidence.
• 1, 2 and 3: Incorrectly includes statement 1 and omits statement 4 (the requirement to state the time unit), which is a core feature of expressing any rate.
• 1, 2 and 4: Incorrectly includes statement 1 and omits statement 3 (defined population), which is necessary since incidence rate is always calculated relative to a specific population at risk.

Thus, the statements that correctly describe incidence rate are 2 (specified time period), 3 (defined population), and 4 (unit of time in the final expression) — corresponding to option 2, 3 and 4.