Cumulative Incidence

Cumulative incidence provides an estimate of the risk that a person will experience an event or develop a disease during a specified period of time. It is calculated as the number of new events or cases of disease that develop in a population at risk during a specific time interval. Cumulative incidence (also referred to as cumulative incidence or incidence proportion) allows researchers to predict risk of a disease or event over a year, 10 years, or even a lifetime; this time period must always be specified when the cumulative incidence is reported.

One example of cumulative incidence is the risk of developing flu among seniors vaccinated against influenza. Another would be the proportion of passengers who develop gastroenteritis while vacationing on a commercial cruise ship for a week. A third example would be the proportion of patients who develop postoperative complications within 1 month of surgery. Individuals in each of these examples meet both the following criteria: (1) They are free of the outcome (influenza, gastroenteritis, or postoperative complications) at the beginning of the study period, and (2) they have the potential to develop the outcome of interest during the study time period.

In the influenza example above, seniors in a study are vaccinated at the beginning of flu season, before any influenza cases arise in the region. There are two ways for the investigators to define the flu season: as a time period (e.g., November to April) or by a combination of a time period and observed events. For instance, in the United States, the flu season is the time period between the first influenza case in the area and the last influenza case in the area during one continuous period of time between September and June. However, the study period is defined; it is the same for all participants in the study, and they all have the same opportunity to be identified as [Page 242]infected with influenza if they should contract the disease.

Cumulative incidence is a useful tool to provide information about the risk of an outcome during a prescribed period of time. In studies where a group is followed for a short period of time, it is possible to compute cumulative incidence directly. For studies where longer follow-up periods are needed, such as in cohort studies of diet and the risk of diabetes, it is not usually possible to estimate cumulative incidence directly. Rather, this question is addressed through the computation of incidence rates. However, rates characterize disease incidence for a group, whereas cumulative incidence characterizes the accumulated risk over time.

From a clinical perspective, cumulative incidence is helpful to public health professionals and clinicians because it can personalize the risk of developing a disease or condition over a period of time that is meaningful to the patient. For instance, a pediatrician might describe an overweight child's likelihood of developing type 2 diabetes in the context of the next 10 years, or by adolescence. While cumulative incidence cannot be computed directly in studies with long follow-up periods due to loss to follow-up, it can be estimated in such studies by first calculating the incidence rate and then estimating the cumulative incidence from the rate. In this case, rates should be constant throughout the course of the study, and if they are not, distinct rates must be calculated for discrete time periods and then aggregated to obtain the best estimate of the cumulative incidence.

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