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The proportion of new cases of the target disorder in the population at risk during a specified time interval. It is usual to define the disorder, and the population, and the time, and report the incidence as a rate.

For some examples of incidence studies, two of the best relate to how Parkinson's disease incidence varies with latitude, and how Perthes' disease (a developmental problem of the hip joint affecting younger children) varies with deprivation.