Prevalence is the number of cases of a disease or condition among a defined group of people during a specific
time period. For example, one can estimate the prevalence of ASDs among 8-year-olds in 2006 in Atlanta,
Georgia, by counting all of the 8-year-olds in Atlanta who were identified with an ASD, and then dividing that
number by the total number of 8-year-olds in Atlanta during 2006. By monitoring ASDs in the same way over
time, we can find out whether the prevalence of autism is increasing, decreasing, or staying the same.
Each year, special education programs in the United States are required to report how many children receive
services for an ASD. From 1998 to 2007, the number of 6- to 21-year-old children receiving services for an
ASD in public special education programs increased from 54,064 to 258,305.4
Those numbers likely did not
include all children with ASDs, because some children received special education for a particular need, like
speech therapy, and not for a diagnosis of autism