Relationship between incidence and prevalence

Incidence and prevalence are closely related. Incidence measures the frequency of events, such as the onset of illness. Prevalence measures the proportion of people who have the illness at any one time.

Example: Prevalence of smoking cigarettes among adults aged 16 and over, 2000 chart

We can represent the relationship using time lines. The horizontal line represents time and the vertical line represents 'now'. Each individual we want to study occupies a position on the chart. A thick horizontal line begins with the onset of disease and ends when the subject no longer has the disease. People free of disease are represented by the absence of thick lines.

The number of thick lines that have started in a known period measure incidence. The number of lines intersected by the vertical represent prevalence. The relationship between incidence and prevalence depends on the number of lines there are and what duration they represent.

Prevalence = incidence x duration.