Incidence (epidemiology)

The incidence of disease is defined as the number of new casesof disease occurring in a populationduring a defined time interval. The number is useful to epidemiologistsbecause it is a measure of the riskof disease.

• The incidence rate is defined as the incidence divided by the sum of the different times each individual was at risk of the disease.
• The incidence per 1,000 is defined as follows:

<math>\frac {\mbox{Number of new cases of a disease occurring in a specified period of time} \times \mbox{1000}} {\mbox{Number of individuals at risk of developing the disease during that time period}}

</math>

Including the number of individuals at risk in the denominatormakes this measure the most common way to express incidence, although other coefficients such as 100,000 are often used.

Incidence and incidence rate are not to be confused with prevalence, which is defined as the number of individuals with a certain disease in a population at a specified time divided by the number of individuals in the population at that time. This measure differs from incidence in that it does not convey information about risk.

To illustrate, a disease with a long duration that was spread widely in a community in 2002will have a high prevalence in 2003(remembering that it has a long duration) but it might have a low incidence rate in 2003. Conversely, a disease that is easily transmitted but has a short duration may have a low prevalence and a high incidence. Incidence rate is useful for talking about diseases like chickenpox, which have a lifetime riskof almost one, since it is measured per unit time so can tell us when infections are likely to occur.

Incidence of disease can also be referred to as absolute risk.

See also

• Prevalencefr:Incidence (épidémiologie)

it:Incidenza lt:Sergamumas

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It uses material from the http://en.wikipedia.org/wiki/Incidence+%28epidemiology%29 Wikipedia article Incidence (epidemiology).