Lead Author(s): Jeff Martin, MD
Measuring Disease Occurrence
Occurrence of disease is the fundamental outcome measurement of epidemiology
Occurrence of disease is typically a binary (yes/no) outcome
Occurrence of disease involves time
Prevalence versus Incidence
PREVALENCE counts existing disease diagnoses, usually at a single point in time
INCIDENCE counts new disease diagnoses during a defined time period
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 can 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.
Prevalence is often more important from the public health perspective of examining the burden of a disease in a population. Incidence is also important to public health in determining trends over time in controlling a disease, but it is the fundamental measure for studies of causality.