Table 1.

Definitions of epidemiological measures

Measure	Definition
Incidence rate	Number of new cases of a disease in a population during a given period of time divided by the total of the lengths of time that each individual in the population was at risk, expressed as person-time (e.g., person-years)
Cumulative incidence	Number of new cases of a disease in a population during a given period of time divided by the total number of people at risk of developing the disease at the beginning of the same period of time
Lifetime risk	Lifetime risk is a special case of cumulative incidence in which the period of time studied is the entire remaining lifetime; if one calculates the entire remaining lifetime from birth, the measure is called lifetime risk at birth
Point prevalence	Proportion of people in a population who have a disease or condition at a particular time point
Birth prevalence	Number of cases (including birth defects among live births, spontaneous fetal death, and induced terminations when available) divided by the total number of live births (or live births plus stillbirths)