Search This Blog

Monday, December 3, 2018

Why Autism (Prevalence) at 2.5% Isn’t Surprising

From lbrb - Left Brain Right Brain
Autism news, science and opinion.

By Matt Carey
November 29, 2018

Let’s get one thing out right away–autism prevalence studies undercount. Not all autistics are diagnosed. That’s just a fact.

Consider the recent CDC study. They look at school and medical records. In many cases, they find children are autistic based on their records–but the schools and doctors hadn’t diagnosed those children.

Combining data from all 11 sites, 81% of boys had a previous ASD classification on record, compared with 75% of girls (OR = 1.4; p<0.01).

Yeah, more than 20% of the kids counted in their prevalence had no diagnosis. They and their families didn’t know.

And, if there isn’t enough in the records to show a kid is autistic? That kid gets uncounted altogether.

So, when people look at the CDC prevalence estimates from over the years and cry “epidemic”, well, there’s a reason why those people usually have some causation theory that they believe in. The irony is that they are usually wrong that their theory needs an epidemic to support it. But, heavily biased people are not usually the best sources of reliable analyses.

What would be a better method of counting how many autistics are in a population? Sounds obvious–test all the kids in a given population. Equally obvious–this is a much more expensive and difficult task. One such study was published in 2011. Yes, 7 years ago.

In Prevalence of Autism Spectrum Disorders in a Total Population Sample, the autism prevalence in Korea was found to be 2.63%. A study performed in South Carolina and reported at IMFAR last year found a prevalence of 3.62%.

This all said, we had another autism prevalence come out this week–The Prevalence of Parent-Reported Autism Spectrum Disorder Among US Children. This study found a prevalence of 2.5%.

Now here’s a nice thing about this recent study–OK, two nice things. First, they don’t just look at kids of one age. Second, you can obtain the data. Which I did. Let’s look at the autism prevalence broken down by birth year.


Do you see autism prevalence increasing with birth year? I don’t. I see some scatter, but in general the autism prevalence is about 2.5% from birth year 2000 to 2010. For what it’s worth–the scatter is due to the small numbers of kids in each year making the value uncertain. It’s statistical noise.

If you are wondering about how the autism prevalence drops off above birth year 2010, keep this in mind: kids aren’t diagnosed at birth. In the CDC study, 1/2 of the kids didn’t get diagnosed until after 52 months (4 years 4 months) of age. For this type of reason, the more recent study didn’t count kids under 3 years of age.

People are very fond of graphing autism prevalence data from various years and claiming these are accurate, full counts of autism prevalence (they aren’t) and, from that, claiming an epidemic.

Here are the CDC data:


The numbers go up. Steadily up. I’d have to be a total denialist to not see that as evidence of an epidemic, right?

Consider this–the CDC autism prevalence for birth year 2000 is 1.1%. The study just out gives an autism prevalence more than double that (2.9%). For the same birth year. Both are good studies, for what they are. Both are limited. But, for one thing, the CDC study was performed in 2008. Ten years ago.

Since then a lot has changed. For one thing, the kids got older and had more chances to get diagnosed. They didn’t just suddenly become autistic in the past 10 years.

So, yeah, we have an autism prevalence estimate of 2.5%. I’m not surprised and I’m not taking this as evidence of an epidemic.

The unfortunate thing in this discussion is that with all the work in this study, all the potential for advocacy, the only number that usually gets discussed is the overall prevalence. Watch the video abstract (which I can’t get to embed). One of the authors goes into a lot of detail about the other findings. Findings I hope to discuss soon.

No comments:

Post a Comment