Stratified Results: ASD and SESa Prevalence Ratios (95% CIb), Stratified by Gender, Pre-existing Diagnosis, Co-occurring Intellectual Disability, Developmental Regression, and Data Source.

aSES indicator is % above poverty level based on United States Census 2000 block group data.bCI = Confidence Interval.cPoverty areas include United States Census 2000 block groups in which more than 20% of families with children have incomes below the poverty level [38]. Percent of cases living in poverty is 20.4% in sites accessing only data from healthcare sources.dIn addition to the χ2 test of the SES gradient within the stratum of children with no pre-existing ASD diagnosis, a separate χ2 test of the difference in the SES gradient for children with and without a pre-existing ASD diagnosis also resulted in a p-value eRestricted to sites with access to school records (n = 2713), including those in Arkansas, Arizona, Colorado, Georgia, Maryland, North Carolina, New Jersey, South Carolina.