Incidence Proportion = Cumulative Incidence = Risk

[Last update: 10/15/03]

Incidence proportion is the proportion of people at risk who develop disease over a stated period of time. Synonyms for incidence proportion include cumulative incidence and risk:

                       no. of disease onsets
Incidence proportion = ----------------------
                           no. at risk

If we start with 245 people who are at risk and 6 develop the disease, the incidence proportion = 6 / 245 = .0245, or 2.45%.

Notes:

• Calculation of incidence proportion requires both a numerator and denominator. 
• The numerator is a subset of the denominator. Thus, this is a mathematical proportion.
• The numerator is the number of disease onsets. 
• The denominator is a count of the number of people at risk of developing the disease at the beginning of follow-up. 
• Incidence proportion requires the follow-up of individuals over time, and is thus longitudinal. 
• Incidence proportion can be calculated only in a closed population (cohort). It can not be calculated in open (dynamic) populations. 
• Incidence proportion is often expressed with a population multiplier (e.g.,.0245 = 24.5 per 1000). See "prevalence notes" for details.
• Incidence proportion = risk. Examples of common risks are listed on Table 6.1 (p. 128).