Assumptions: Cumulative Incidence and Incidence Rate

Lead Author(s): Jeff Martin, MD

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.

Assumption in Follow-up Starting Times

It should be clear that by assigning everyone the same starting time you are making the assumption that there are no calendar time trends that will affect your estimate of incidence. You have dropped calendar time, and hence trends associated with calendar time (often called secular trends), out of the analysis. For some situations, though, this may be a dubious assumption. For example, if one were enrolling subjects to study a new infectious disease during its introductory epidemic period (HIV, SARS, ebola, etc.), temporal trends might affect incidence estimates significantly.

Assumptions for Cumulative Incidence and Incidence Rate Calculations

Note that Szklo and Nieto in the table below use the headings Survival Analysis for cumulative incidence approach and Person-time for the incidence rate approach in analyzing data that features time to an event. These are the assumptions of both methods.



Szklo, M., & Nieto, F. (2007). Epidemiology: Beyond the Basics (2nd Edition ed.). Boston: Jones and Bartlett Publishers.