- The CDC has reported that the prevalence of autism among children born in 1994 is 1 in 150.
- The administrative prevalence of autism among 6 year olds, as reported in IDEA, has increased 124% (factor of 2.24) from 2000 to 2006.
- Therefore, they argue, the prevalence reported by the CDC must actually be 1 in 67 for children born in 2000, if adjusted accordingly.
Anyone see why this methodology does not hold?
For this analysis to hold, IDEA ascertainment would have to be at least roughly equivalent to CDC ascertainment. It's not a matter of "not believing the statistics collected by the U.S. department of education" as the authors contend. I believe them. I just don't think they are counting all autistic children, by far. But I do think they are getting better at ascertaining autism.
In 2000, the prevalence of IDEA autism among 6 year olds was 0.23% (source, table B2C). This is 2.89 times less than the CDC prevalence reported for children born in 1994. So for every 3 autistic children, IDEA missed 2. Of course, it's unlikely they missed most of them. They were probably served under other categories, as demonstrated by Shattuck (2006).
Could it also be that the IDEA category can be changed after the age of 6? (Note that in IDEA a child is in one category or another, not multiple ones simultaneously.) The IDEA prevalence of autism among 11 year olds in 2005 was 0.47% (same table), more than double that of 6 year olds in 2000. Obviously, half of the autistic 11 year olds, as recognized in 2005, must have been served in other categories in 2000.
So Gallup & Yazbak's analysis is flawed in regards to using the IDEA prevalence of 6 year olds in 2000 to extrapolate the results of a survey done recently on children who are now older. That's just a demonstrable flaw of the analysis. I don't believe the analysis methodology is valid either way.