What is the difference between point and period prevalence?

• A. Point prevalence is at a single time point; period prevalence is over a time interval
• B. Point prevalence is over a time interval; period prevalence is at a single time point
• C. Point and period prevalence are the same concept in surveillance
• D. Point prevalence is always higher than period prevalence in an outbreak

Answer: (D)

The difference being tested is how prevalence is measured over time. Point prevalence is the proportion of a population that has the disease at a single, specific moment. Period prevalence, on the other hand, is the proportion of the population that has had the disease at any time during a defined interval. In an outbreak, period prevalence collects every case that occurred during the entire period, while point prevalence looks at who is ill at one precise time within that period. Because period prevalence includes all cases that happened over the interval (including those who recovered before the end or are still ill at the end), it is not restricted to a single moment and can be larger than the point prevalence observed at any single time point. The statement that point prevalence is always higher is not correct. The correct distinction is that one is a snapshot at a single time, the other a count over a time span. For example, if ten people became ill over two weeks but only three are ill at the end, period prevalence would reflect ten cases, whereas the point prevalence at the end would reflect three.