Incidenca

Incidenca, commonly rendered as incidence, is a measure of the occurrence of new cases of a disease or condition in a defined population over a specified period. It is a central concept in epidemiology and public health, used to quantify risk and to monitor trends. Incidence is distinct from prevalence, which counts all existing cases at a given time rather than new ones.

Two main forms are used in practice: cumulative incidence and incidence rate. Cumulative incidence (risk) equals the number of new cases during the period divided by the number of individuals at risk at the start of the period, and is usually expressed as a proportion or percentage. Incidence rate (incidence density) equals the number of new cases divided by the total person-time at risk, and is typically reported per 1,000 or 100,000 person-years. Calculations require clear definitions of the population at risk, the time window, and how cases are ascertained. Data sources include cohort studies, disease registries, and surveillance systems.

Interpreting incidence depends on context. Estimates can be influenced by the size of the population, the length of the observation period, and the completeness of case reporting. Incidence can be stratified by subgroups such as age or sex to reveal differences in risk. In broader usage, the term can also appear in mathematics or statistics to describe the relation or frequency with which events occur, or in geometry as an incidence relation between objects. Etymologically, it derives from Latin incidere, “to fall upon.”