That's the proportion of school-aged kids found to meet diagnostic criteria for an ASD by this team of mostly Korean researchers --- Drs. Young-Shin Kim, Bennett L. Leventhal, Yun-Joo Koh, Eric Fombonne (who has done a lot of research on the prevalence of autism), Eugene Laska, Keun-Ah Cheon, Soo-Jeong Kim, Young-Key Kim, Dong-Ho Song, and Roy Richard Grinker (the Unstrange Minds guy), along with Eun-Chung Lim and HyunKyung Lee --- when they screened elementary-school children in one district of a fairly large city (Goyang City, pop. 1,073,069; Ilsan district, pop. 488,590) near Seoul, South Korea.
Part of their reasons for doing this study (full text here) was their thought that previous estimates of the prevalence of autism rely too much on documentation from official sources --- national disability offices, special-education registries, autism-specific service providers, etc. --- that wouldn't include children who have not yet been evaluated for ASDs.
To try and get more of a cross-section of Korean children, the researchers recruited from both mainstream elementary schools (41, of which 30 agreed to participate), special-education schools (3), and the city's Disability Registry (all children between 7 and 12 listed as having either ASD or intellectual disability). Children from participating institutions were initially screened for autism using the Autism Spectrum Screening Questionnaire (ASSQ), which parents and/or teachers filled out about each child.
Not surprisingly, the general-population group, recruited from the elementary schools, was much larger than the developmentally-disabled group recruited through the Disability Registry or from special-education schools: in all 30 participating elementary schools, there were 36,592 children between the ages of 7 and 12. Of these, the parents of 23,234 completed the ASSQ and sent it in to the researchers; of the 294 students attending special schools and/or listed in the Disability Registry, 103 had forms submitted for them.
Anyway, following this initial screening, the students who scored high enough on the ASSQ (with a teacher-rated score of at least 10, or a parent-rated score in the top five percentiles*) were offered further diagnostic evaluation. Among the elementary-school students, 1,742 met those screening criteria, and 234 completed the evaluation. All of the special-education students were considered to meet screening criteria, and 52 of them completed the evaluation. Each child was evaluated by two teams, and each team included both Korean- and American-trained practitioners. The evaluators used Korean versions of the Autism Diagnostic Interview (ADI-R), Autism Diagnostic Observation Schedule (ADOS), and two cognitive tests, the WISC-III and a revised version of the Leiter International Performance Scale.
Of the 286 children who were evaluated, 201 met criteria for an ASD**. Just over half of these (101) met criteria for Autistic Disorder; just under half of those (48) had come from the special-education group. All but one of the students in that group who were diagnosed with an ASD were diagnosed with Autistic Disorder, while most of the students in the general-population group were diagnosed with some other ASD, either Asperger's syndrome or PDD-NOS**.
There is one thing I found particularly interesting in their results: the ratio of boys to girls among their ASD "positives" is significantly lower than the 4:1 most commonly cited in the autism literature. These researchers found a ratio of 5.1:1 among the special-education students (who were mostly boys to begin with --- of the 103 special-education students who were screened, 84 were male, so even if *all* of the girls in that group had autism, the ratio would still be skewed in the boys' favor unless relatively few of the boys had it) and a ratio of 2.5:1 among the mainstream elementary-school students.
That number adds some support to the hypothesis that autism remains underdiagnosed among girls --- the lower ratio was only seen in the students who were not flagged as disabled or special-ed, not receiving any services. Among the students known to be developmentally disabled, or already diagnosed with autism (i.e., those registered as ASD in the Disability Registry), the ratio was closer to the current conventional-wisdom ratio of 4:1.
(It also seems like the forms of autism that are most likely to go undiagnosed in girls are the "milder" forms, like Asperger's and PDD-NOS. Very few girls in the general-population sample met criteria for Autistic Disorder; of the 27 children meeting those criteria, only five were female.)
Anyway, back to the number making up the title of this post.
How they got that number is complicated. They divided the number of autistic students they extrapolated there to be based on the number they found (more about this later) by the total number of seven-to-twelve-year-old children living in the Ilsan district of Goyang City, which is 55,266.
So much for the easy part.
There were several steps involved in correcting for the large proportion of nonresponders; first, and easiest, they guesstimated the likely full scores of the people who turned in partial ASSQs. (The simplest method they had for doing this was simply to take the average single-item score on a form in which only some of the items are answered and multiply it by the number of items on the full questionnaire, which is 27). Just doing that gave them 264 more subjects meeting initial screening criteria for an ASD. They also added in all the children listed in the Disability Registry as having ASD who did not participate in this study; this would add 101 subjects to the confirmed-ASD pool. (They left out all the non-participating children listed as having intellectual disability).
They also did some statistical manipulations that I don't understand (I had minimal coursework in statistics, and not even good old Google can tell me what a "weigh-back procedure"*** is) to correct for their only having given some of the people meeting their minimum screening criteria the opportunity for furthere evaluation. (See the Methods section and this supplemental section for their discussion of these techniques).
At the end of all of this, they ended up with an adjusted prevalence figure of 2.64%, or the 1 in 38 referred to in the post title. There are several ways in which this might be an overestimate: chief among them, the researchers assumed that the proportion of children meeting screening criteria who also meet diagnostic criteria for an ASD would be the same for the nonresponders as it was for the responders --- i.e., about 70%. Their basis for this assumption is a logistical regression model they created to try and predict parental consent to, and participation in, diagnostic evaluations. They found that since none of the co-variates they'd chosen for this missing "latent variable" --- child's age, sex and ASSQ score --- predicted whether a given child would end up with an ASD diagnosis or not, that parental willingness to have more tests done did not vary with the likelihood that their child is autistic. But all of those variables are somewhat crude proxies --- even the ASSQ score, when you consider that all of the "screen-positive" children had scores clustered within the same narrow window, and thus variation between them would be minuscule. So I think it's reasonable to think that maybe the model was wrong, and the parents who chose to have their children evaluated might have a higher proportion of autistic children than the parents who opted out.
However, there are just as many reasons to suspect that their numbers are an underestimate --- after all, they did write off all the nonparticipants labeled intellectually disabled, even though their own results show a substantial proportion of intellectually-disabled responders having autism. The authors also suspect underreporting of possible autism by both parents and teachers, because autism is stigmatized in South Korea (thus making parents less likely to want to admit that their child has it) and because the South Korean school day is so rigorous and structured (making teachers less likely to notice the social dysfunction that the ASSQ tests for, because there is so little socializing during a typical school day in Korea).
But the need to extrapolate so much data to make up for low response rates is a serious flaw in this study, though, as Prometheus points out, even when you restrict your gaze to the data that they actually have, you still see that the majority of confirmed ASD cases were in the general-population group, among children who had never seen a mental-health professional before.
Other bloggers posting about this study: Steven Novella at Science-Based Medicine; Sullivan at Left Brain/Right Brain and the Autism Science Foundation blog; Rose at Hard Won Wisdom; Michelle Diament at Disability Scoop; Paul Whiteley at Questioning Answers; Polly Palumbo at Momma Data; Aspie Editorial; Raphael Fraser at Music, Medicine & the Mind; Prometheus at A Photon in the Darkness; and MJ at Autism Jabberwocky (who has a detailed explanation of the ASSQ and its inherent biases)
*Only some of these children were referred for further evaluation: all the children scoring in the top two percentiles, half the children (chosen randomly, not by score) in the percentile third from the top, and one-third of the children in the percentiles fourth and fifth from the top.
**There's another interesting asymmetry within the general-population-derived ASD cases: a much greater proportion of those whose final diagnosis was Autistic Disorder had histories of psychiatric or psychological treatment. A majority (26 of 48) of students attending mainstream elementary schools who both met criteria for an ASD and had seen a psychiatrist or psychologist in the past ended up being diagnosed with Autistic Disorder, while the ASD students from mainstream elementary schools who had never seen a mental-health professional were mostly diagnosed with the other ASDs (27 of 104 with Autistic Disorder; 77 of 104 with either Asperger's or PDD-NOS).
***Perhaps it is how Mr. Peabody travels back in time?
Kim, Y., Leventhal, B., Koh, Y., Fombonne, E., Laska, E., Lim, E., Cheon, K., Kim, S., Kim, Y., Lee, H., Song, D., & Grinker, R. (2011). Prevalence of Autism Spectrum Disorders in a Total Population Sample American Journal of Psychiatry DOI: 10.1176/appi.ajp.2011.10101532