The difference between incidence and prevalence is a fundamental distinction in epidemiology. Prevalent cases of a disease will over-represent those with longer duration or survival. This can potentially introduce significant bias into a study of a disease and its risk factors. The amount of the difference between incidence and prevalence is related to the time period during which incidence is measured and the average length of duration of the disease condition. In some instances prevalence may look similar to incidence and in others the two measures will be very different.

The concepts are not difficult to grasp but there are some subtleties in implementing them as diseases with gradual onset can be diagnosed at varying points in their development, cancer being the most common example. Both incidence and prevalence can be affected by changes in methods of diagnosis and the ability to identify disease at earlier stages.

Incidence is also important to public health in determining trends over time in controlling a disease. It is the fundamental measure for studies of causality.