Period prevalence versus point prevalence formula: which is true?

• A. They are identical
• B. Period prevalence uses average population during period; point prevalence uses population at a point
• C. Period prevalence uses population at start only
• D. Point prevalence uses average population across the period

Answer: (B)

Prevalence measures how much of a population is affected by a condition. Point prevalence gives the proportion of people who have the condition at a single moment in time, using the population at that exact moment as the denominator. Period prevalence, on the other hand, aims to capture all cases that exist at any time during a defined interval, and it uses the average population during that period as the denominator to reflect how many people were at risk across the span. This makes sense because the population can change over the period, so the mid-interval or average population better represents the typical population exposed to the condition during the whole time window. The other statements don’t fit: point prevalence is not tied to the start or end population alone, period prevalence is not limited to the start population, and the two measures are not identical.