Friday, May 08, 2009

Another Way to Tell the "Epidemic" in California is Bogus

There's been some renewed California "epidemic" talk recently because of a report released by California DDS (this one) despite the usual DDS disclaimer to the effect that report numbers don't represent epidemiological counts of all autistic persons in the state. Kristina discussed it and there's been some media coverage as well, with the usual Rick Rollens scare-mongering about an upcoming surge of autistic adults that the state of California allegedly doesn't have any experience dealing with. There are a number of inaccuracies and faulty assumptions in the media coverage of the report, to be sure, but I wanted to focus on one particular claim found here:

The percentage of people with both autism and mental retardation has dropped significantly, a trend that may provide clues for those trying to solve the autism puzzle.

That's not stated accurately, but in fact there has been a gradual drop in the prevalence of mental retardation within the population of recognized autistics, at least for the last 17 years. See page 20 of the report.

This is a well known fact, or at least I'd like to think it is. I've discussed it several times previously (here, for example.) It is mentioned in Gernsbacher et al. (2005) as well.

I'm sure broadening ascertainment denialists can find ways to rationalize this finding. But what if I told you that the phenomenon is not only a time-based phenomenon? It can also be observed when you compare one regional center to another. That is, regional centers with a higher administrative prevalence of autism will tend to have a lower proportion of autistics who also have a classification of mental retardation.

You know a chart is coming, but as usual I'd like to be clear as to where the data comes from, so anyone reading can double-check if they so wish. I'm using a file provided by California DDS upon request, named It only goes up to January, 2006, but it contains more autism-specific information for each regional center than you normally find in the regular report. I don't have population data for each regional center (and gathering that would be a bit much for a blog post) but I will use the Autism-Epilepsy ratio as a proxy of administrative prevalence. The administrative prevalence of epilepsy in California is roughly stable (just below 0.1%) so dividing the autism caseload by the epilepsy caseload of a regional center should provide us with an adequate proxy of the administrative prevalence of autism.

The figure demonstrates an inverse association between the Autism-Epilepsy ratio and the proportion (%) of autistics who also have an MR (or unkown MR) classification. The downward trend is statistically significant with 99.6% confidence.

Now, you'll note the distribution of the dots in the chart is fairly random. I wondered why that might be, especially why the Central Valley regional center would have a low administrative prevalence of autism and a low proportion of autistics with MR.

The Central Valley RC has 9,284 clients with mental retardation as of January, 2006. That's comparable to other big RCs. Of all individuals with MR, only 247 (2.7%) also have an autism classification. This is ridiculously low. I'm sure that if someone went to the Central Valley RC and screened the individuals with an MR classification, they would find that a lot more than 2.7% of them are also autistic.

It's generally understood that there's been increasing recognition of autism in the population without mental retardation. This is what they call HFA. What's not so intuitive is that there's also been increasing recognition of autism in the population with mental retardation. Clearly, regional centers have different levels of recognition across both populations. In average, 7.7% of persons with mental retardation will have a classification of autism in California (which is still rather low.)

Let's look at what the result would be if we were to adjust the autism numbers under the assumption that every regional center should have a level of recognition of 7.7%.

That's a lot more clear, isn't it? This suggests that differences in the recognition of autism in the population with MR across regional centers are an artifact.

Thought Experiment

Imagine there's a hypothetical regional center where recognition of autism has gotten out of hand, to the point where every person who resides in the area served by the RC is classified as autistic by it. In this case the Autism-Epilepsy ratio would be about 1,000.

You'll note the first chart above has a power regression model (the formula on the upper right.) The type of model is theoretically justifiable, and I can discuss that on another occasion.

y = 32.235 x-0.4525

In the model, x is the Autism-Epilepsy ratio and y is the proportion of autistics with MR.

So what if x is 1,000? This model predicts that the proportion of autistics who have MR would then be 1.41%. This is basically what you'd expect for the general population. In other words, DDS data is entirely consistent with a cultural explanation of the differences in administrative prevalence of autism between regions.


  1. I looked at the blue dots and saw that the lower the number with mental retardation/intellectual impairment (70% and 0.6 autism-epilepsy ratio as against 30% 1.8 [?]) the lower there would be with epilepsy too.

  2. I'm not sure I follow that, Adelaide, but the proportion of autistics with epilepsy has also dropped over time, yes. This is unrelated to the total epilepsy caseload in the system, though, which is roughly stable.

    The autism-epilepsy ratio is just an approximation, but it is in fact the case that regional centers in the LA area have a higher administrative prevalence than the rest of the state.

    Checking the population of some counties, the admin. prevalence of autism in the Redwood Coast RC would be 6.07 in 10,000. Aggregating all LA county area regional centers, the admin. prevalence in LA is 12.3 in 10,000. Basically twice.

    The admin. prevalence in the Central Valley RC is 4.26 in 10,000. So about a factor of 3 from lowest to highest.

  3. SIDS, Autism, Autism Spectrum Disorder, Plagiocephaly, etc. are all
    conditions that the medical profession is trying to treat.
    The SIDS back sleep (Supine) sleep recommendations began in 1992
    The SIDS "Back to Sleep" campaign began in 1994.
    THe Netherlands began their SIDS Back to Sleep Campaign in 1987.


    Also see:

    Individuals with Disabilities Education Act (IDEA) Data website:
    Pregnancy Risk Assessment Monitoring System (PRAMS) sleep position data:
    Centers for Disease Control (CDC) Birth Data:
    NOTICE PDR-2006-01

    Gastrointestinal Disorders also known as GER is a common comorbidity

    Maternal smoking decreased significantly between 1990 and 2002

    Here is a good article on diagnosing this:

  4. Hello- I am sorry for this post being off topic but I wanted to bring to your attention a survey I designed in order to evaluate the quality of life a sub-group of people with autism have (ASD people able to communicate in writing).

    This is as a result of having had some much discussion lately on what ND and ND HFA/AS wish for, and not actually knowing exactly how representative the various opinions.

    Many thanks for your time.
    The data will be collected at a later stage. The analysis of this survey will be presented on upon completion. This data may be used as pilot data for a more accurate survey of ASD adult population.