Prevalence refers to the rate of the disorder present in the population at a given point in time.
Incidence refers to the number of new cases occurring in the population during a given period.
There are no prevalence estimates specifically for the United States, but recent studies from Canada and from Japan indicate that autism is not a rare disorder. Both studies found prevalence rates of autism greater than 10 per 10,000. A rate of at least 22 per 10,000 has been estimated for the broader autism spectrum disorders. Because of similarities between the United States and Canada, the Canadian data are likely to be reasonable estimates of prevalence in the United States for most purposes. Given the available data, there is little justification for the potential costs of a national prevalence study of autism in the United States merely to estimate the prevalence of autism spectrum disorders.