Which statement best differentiates point prevalence from period prevalence?

• A. They are the same
• B. Point prevalence is the proportion ill at a single point in time; period prevalence covers a time interval
• C. Point prevalence uses incidence; period prevalence uses mortality
• D. Point prevalence is always higher than period prevalence

Answer: (B)

The key idea is how time is treated in measuring prevalence. Point prevalence gives the proportion of people who have the condition at a single moment—like a snapshot on a specific day. Period (or interval) prevalence, on the other hand, covers a defined time span and includes everyone who has the condition at any time during that period, typically in relation to the population at risk during that period. That difference in time framing is what distinguishes the two.

So, distinguishing statements: point prevalence is the proportion ill at a single point in time, while period prevalence covers a time interval. For example, counting everyone with asthma on the census day yields point prevalence; counting everyone who had asthma at any time in the past year yields period prevalence. The other ideas—linking point prevalence to incidence, or to mortality, or assuming one is always higher than the other—don't correctly capture how these prevalence measures are defined.