Saturday, October 01, 2005

Reviewing the Autism Prevalence (The Use of the IDEA Data: Part 2)

A major concern with the IDEA autism numbers is that they are different from the epidemiology that exists for autism. I mentioned some of these differences in the last post and will add to this in the current post. Such differences are important as they show that the IDEA based autism numbers are not meant to calculate prevalence.

I want to point out that the education numbers are important. They can show emerging trends (if not actual incidence) and are useful if we need to decide to allocate more funding to this or that area, or urge more Universities to start a autism certification program for their student special education teachers. As they are different from the mental health based DSM-IV diagnostic system, they inevitably measure different things.

I have mentioned before that some groups persist in using these numbers to show prevalence, and that this is not a good idea. However, if we insist on using the IDEA numbers and that they show an epidemic, we could make a number of predictions. We can base these on other times a given population increased. Take fore example the US population under the baby boom (The great increase of babies being born just following World War II). If we were to graph all ages from 1947-1980, we would see a curve or a wave, slowly travel across the years as the baby boomers advanced in age. We could expect to see something similar in IDEA numbers.

I included some graphs I made that show the number of students receiving services in the autism service category by age from 1996-2003.

The traveling wave, is absent. The most notable trend, is an increase year to year at nearly every data point as noted by Laidler (2005). I included a second chart with a red line to show the increase as six year olds become seven year olds and so on.

Analyzing the trend is somewhat difficult because there seems to be no one receiving services for autism below age 6 in the years prior to 2001. I know several children who were below age 6 and were diagnosed in the years prior to 2001, I would assume an awful lot of people do.

By why the increase from corresponding data points? We would assume that there would be nearly the same number of 7 year old in 2001 as we had 6 year olds in 2000. That is not what we see. This brings us into the second major trend in the graph; the lines follow a very similar pattern of increase, plateau, and then decrease. This indicates that the line form is a function of another factor besides actual prevalence.

We know that students do not first demonstrate the characteristics of autism at age 13, so why the increase from the 12 year olds the previous year?

Two things come seem possible, either there is a progression to autism that can occur much later than indicated in the DSM-IV or we missed them earlier and are playing catch up by diagnosing now. But remember the IDEA categories do not constitute a diagnosis, simply an assignment to a service category.

And that is the answer I think. We simply assign to the category as necessary for services, and we have been increasingly doing so since 1990 as the service category becomes more accepted and used (Gernsbacher, Dawson, Goldsmith, 2005). As the students leave school, or enter private school, or are reassigned to some other service category as they grow older, they appear to be the lower prevalence of the older children.


American Psychiatric Association. (1994). Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition. Washington, DC: American Psychiatric Association; 1994.

Gernsbacher MA, Dawson M, & Goldsmith HH. (2005).Three reasons not to believe in an autism epidemic.Current directions in psychological science, 14 (2), 55-58.Individuals with Disabilities Education Act. (1990). Public Law 101-476, U.S.C.

Laidler, J. (2005). US Department of Education data on "autism" are not reliable for tracking autism prevalence. Pediatrics, 116 (1), 120-124.

The IDEA data is taken from the and from the links to the various earlier IDEA reports to congress. These can be accessed here and here


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