Period prevalence is defined as:

• A. Incidence rate per person-time
• B. Number ill during a time period divided by average population
• C. Number ill at a point in time divided by total in group at a point in time
• D. Cumulative incidence over lifetime

Answer: (B)

Period prevalence measures the proportion of people who have the disease at any time during a specified period. It is calculated as the number of individuals who are ill at some point during the period divided by the average population during that period. The numerator counts anyone who experienced the illness at least once during the interval, while the denominator uses the average population to account for changes in population size over the period. For example, over a year with an average population of 10,000 and 300 people who were ill at some point during the year, period prevalence would be 300/10,000 = 3%. This differs from point prevalence, which uses the number ill at a single moment divided by the group size at that moment, and from measures of incidence (like incidence rate per person-time or cumulative incidence) that focus on new cases over time. So the description “number ill during a time period divided by average population” is the correct definition of period prevalence.