Cumulative Incidence - Person-Time

Lead Author(s): Jeff Martin, MD

Definition of Cumulative Incidence

The proportion of individuals who experience the event in a defined time period (E/N during some time T) = cumulative incidence


E(vent)/N(umber) during some time T(ime) = cumulative incidence

Time Period for Cumulative Incidence

If the measure is a proportion of persons, it is unitless since it has to vary between 0 and 1. In other words it is a probability.

Cumulative Incidence Measure

Perhaps most intuitive measure of incidence since it is just proportion of those observed who got the disease

Two primary methods for calculating cumulative incidence: Of the two methods for calculating cumulative incidence, the life table method is older but is now seldom used except in actuarial tables of life expectancy and a few similar settings with large numbers of persons. The Kaplan-Meier method is very similar and has become the usual method for estimating cumulative incidence.

Calculating Cumulative Incidence

With complete follow-up cumulative incidence is just number of events (E) divided by the number of persons (N) = E/N

The simplest situation in which to calculate cumulative incidence is if all of the persons are followed for the same length of time.

Outbreak investigations, such as of gastrointestinal illness, typically calculate attack rates with complete follow-up on a cohort of persons who were exposed at the beginning of the epidemic. An example can be found in the GI illness in Livington County.

Unfortunately, the term attack rate has traditionally been used to describe the proportion of persons who develop illness.

Clinical Trial Denominator

Counting all individuals in the denominator can also be found in a clinical trial, which you will recall is formally a type of cohort study.

Specifying Time Period for Cumulative Incidence

Calculating cumulative incidence with different follow-up times, assumes the probability of the outcome is not changing during the study period = no temporal/secular trends affecting the outcome.

Cumulative incidence cannot be interpreted without specifying the time period.

Temporal trends, such as improved survival, can lead to changes in cumulative incidence as seen in the Australian children's cancer study and the SEER cancer monitoring program.

The longer the follow-up period of an analysis, the greater the threat that changes in the underlying incidence rate of the outcome may be causing an estimate of cumulative incidence to be invalid.

Changes in treatment are a common way in which survival analysis may be biased by the assumption of no temporal trends.

There are other changes that can produce temporal trend bias as well, such as: