The autism number everyone quotes is not a national rate

The figure everyone quotes for autism comes from CDC's Autism and Developmental Disabilities Monitoring Network, and its construct label matters more than its value: records-based identification, not diagnosis. ADDM counts 8-year-olds who meet a surveillance case definition in health and, where available, education records, at a non-representative and changing set of US sites. A child never told "you have autism" can be an ADDM case; a diagnosed child whose records are not accessible can be missed.

Significant findings

In surveillance year 2022 the combined figure was 32.2 per 1,000 children aged 8 - "one in 31" - across 16 sites. The per-site figures in that same year ran from 9.7 per 1,000 in the Laredo, Texas area to 53.1 per 1,000 in California, a 5.5-fold spread inside one year of one surveillance system. A national rate does not vary five-fold with the county you happen to measure it in.

The spread also changed shape over time, which is its own tell. In 2016 the 11 sites agreed almost perfectly, 18.0 to 19.1 per 1,000. By 2022 they disagreed by a factor of 5.5. A number whose internal spread behaves like that is telling you about identification systems, not about children.

CDC states the limitation itself, in the limitations section of its own report: "Sites participating in the ADDM Network are selected through a competitive process, and the resulting catchment areas are not designed to be representative of the states in which the sites are located. Findings do not necessarily generalize to all children aged 8 years in the United States, and interpretations of temporal trends are complicated by changing catchment areas, case definitions, and diagnostic practices." The 2022 report adds that the surveillance areas "do not generate nationally representative ASD prevalence estimates." The full annotated series - every surveillance year with its site range and the agency's own comparability warnings - sits at www.resolv.social/topics/how-autism-got-counted.

Worth asking

If a number is quoted to you as "the autism rate," the useful questions are what it counted and where: records or diagnoses, which ages, how many sites, and what the range across those sites was. Ask whether the figure being compared against it came from the same instrument, because most autism comparisons that circulate did not. And if the question underneath is about a specific child, a population statistic is the wrong tool entirely - that is a question for the clinician who can actually assess them.