Incidence is defined as:(AIPG 10)
|A||Number of cases existing in a given population at a given moment of time|
|B||Number of cases existing in a given|
|C||Number of new cases occurring during a specified period|
|D||Number of old cases present|
Incidence - number of new cases occurring in a define population during a specified time period.
Number of new cases of specific diseases during a given time period x 1000
Population at risk during that period
Special incidence rates
1. Attack rate (case rate) -Incidence rate used only when the population is exposed to risk for limited period of time (as epidemic)
Number of new cases of specified disease during a time interval x 100
Total population at risk during same interval
2. Secondary attack rate -Number of exposed person developing the disease within one incubation period following exposure to a primary case.
Uses of incidence rates
- To control disease
- For research into aetiology and pathogenesis and distribution of disease and efficiency of prevention therapeutic.
Option A. would give the prevalence of the disease.