prevalence

Meaning: Prevalence is the proportion of a population that has a particular condition or trait, including both new and long-standing cases. Point prevalence is measured at one moment, period prevalence over an interval such as 12 months, and lifetime prevalence is the share of people who have ever had the condition. It depends on how many new cases arise (incidence) and how long each case lasts. A long-lasting condition can have high prevalence even when incidence is low.

Equation: Prevalence = number of existing cases ÷ number of people in the population. Both terms are counts of people, so prevalence is a unitless proportion, often reported as a percentage or per 100,000 people. When a condition is stable and fairly rare, prevalence ≈ incidence × average duration.

On the MCAT: Social epidemiology passages report prevalence of disorders or health behaviors (depression, substance use, obesity) across groups defined by gender, race, age, or socioeconomic status. You may compare groups, read a table or graph, or predict how prevalence changes. A treatment that extends life without curing the condition raises prevalence. A cure, faster recovery, or high mortality lowers it.

Common confusion: Students treat prevalence and incidence as the same. Incidence is the rate of new cases in a period. Prevalence is a snapshot of all current cases. A new case raises both, but recovery or death lowers prevalence only. Prevalence is also not the same as an individual's risk.