incidence (epidemiology) incidence (epidemiology) in epidemiology, incidence is a measure of the probability of occurrence of a given medical condition in a population within a specified period of time. although sometimes loosely expressed simply as the number of new cases during some time period, it is better expressed as a proportion or a rate with a denominator. incidence proportion incidence proportion (ip), also known as cumulative incidence, is defined as the probability that a particular event, such as occurrence of a particular disease, has occurred before a given time. it is calculated dividing the number of new cases during a given period by the number of subjects at risk in the population initially at risk at the beginning of the study. where the period of time considered is an entire lifetime, the incidence proportion is called lifetime risk. for example, if a population contains 1,000 persons and 28 develop a condition from the time the disease first occurred until two years later, the cumulative incidence proportion is 28 cases per 1,000 persons, i.e. 2.8%. ip is related to incidence rate (ir) and duration of exposure (d) as follows: i p ( d ) = 1 − e − ∫ t = 0 d i r ( t ) d t . ^ir(t)t}\,.} incidence rate the incidence rate is a measure of the frequency with which a disease or other incident occurs over a specified time period. it is also known as the incidence density rate or person-time incidence rate, when the denominator is the combined person-time of the population at risk (the sum of the time duration of exposure across all persons exposed). in the same example as above, the incidence rate is 14 cases per 1000 person-years, because the incidence proportion (28 per 1,000) is divided by the number of years (two). using person-time rather than just time handles situations where the amount of observation time differs between people, or when the population at risk varies with time. use of this measure implies the assumption that the incidence rate is constant over different periods of time, such that for an incidence rate of 14 per 1000 persons-years, 14 cases would be expected for 1000 persons observed for 1 year or 50 persons observed for 20 years. when this assumption is substantially violated, such as in describing survival after diagnosis of metastatic cancer, it may be more useful to present incidence data in a plot of cumulative incidence, over time, taking into account loss to follow-up, using a kaplan-meier plot. incidence vs. prevalence