In incidence measurement, what is the appropriate denominator?

• A. Number of individuals at risk during observation period
• B. Total population
• C. Number of new cases
• D. Number tested

Answer: (A)

Incidence measures new cases arising over a defined time, so the denominator must reflect who could potentially develop the disease in that period. That means using the number of individuals who are disease-free and at risk at the start (or during) the observation period. Using the total population includes people who cannot develop the disease during the interval (because they already have it or are not susceptible), which would distort the rate. The numerator is the count of new cases, not the denominator. The number tested is not a measure of risk in the population and can introduce bias if testing isn’t uniform. If you were calculating an incidence rate in terms of person-time, the denominator would be person-time at risk, but among the given options, the appropriate denominator is the number of individuals at risk during the observation period.