Caption: Incidence Rate
Text: The incidence rate for an exposure group is computed by dividing the number of new events (incident events) by the sum of the disease-free observation times for all subjects in the exposure group. The disease-free observation time for each subject is the time from their enrollment until a) they develop the disease, b) they become lost to follow up, or c) the study ends. In addition, one way of dealing with subjects who change their exposure status (e.g. by quitting smoking) is to stop counting time they contribute to the exposure group once the exposure status changes.