Evidence-appraisal glossary
Incidence
Incidence is the rate at which new cases of a condition appear in a population over a set period. It counts only people who develop the condition during that time, among those who were previously unaffected and at risk. It measures how fast a condition is arising, unlike prevalence, which counts existing cases.
Also called: incidence rate, cumulative incidence, attack rate.
Incidence quantifies how frequently new cases arise in a defined at-risk population over a specified period. Two common forms exist: cumulative incidence (the proportion of an initially disease-free group that develops the condition during follow-up, a risk) and incidence rate (new cases divided by the total person-time observed, which accounts for people followed for different lengths of time). Because incidence captures new events, it is the natural measure for studying causes, risk factors, and the effect of interventions. When reading a study, check who was counted in the denominator (only those at risk and initially unaffected), how long they were followed, and whether the paper reports incidence or prevalence, since confusing the two misleads. For example, if 50 new cases occur among 5,000 people followed for one year, the cumulative incidence is 1 percent per year. Look for clear definitions of the at-risk group and the observation window before comparing incidence figures across studies.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.