incidence

Meaning: In epidemiology and medical sociology, incidence describes how often a disease, disorder, or behavior newly appears. Only new cases count, and the population counted is those at risk, over a stated period such as one year. It is often reported per 1,000 or per 100,000 people.

Equation: Incidence rate = number of new cases during the period ÷ population at risk during that period. New cases are a count (unitless), the population is a count of people, and the period is in seconds in SI terms (years in practice), so the rate is cases per person per unit time.

On the MCAT: Passages on health disparities, social epidemiology, or mental disorders may give a table of rates across groups, such as by social class, race, or gender. You compare groups or infer a cause. A common item asks which measure rises or falls when a condition changes. A new treatment that extends life without curing a chronic illness leaves incidence unchanged but raises prevalence, because patients stay in the population longer. A condition with a short duration, such as a brief illness, can have high incidence and low prevalence.

Common confusion: Students treat incidence and prevalence as the same thing. Incidence counts only new cases over a period. Prevalence counts all current cases (new and existing) at one moment or over a period. Prevalence depends on both incidence and how long the condition lasts.