Autistic Parent Questions If Autism Is Linked To Rainfall
A while back I wrote a critique of the TV hypothesis by Waldman et al. I noted the likely confound is population density, which should not be considered a "fixed effect" in Waldman's methodology (an interesting statistical methodology that is apparently used in Economics frequently). When we talk about population density as a confound, we're really using it as a proxy of other confounds that are clearly not fixed in time. These more specific confounds could be things like awareness, availability of autism specialists, etc.
In general, studies like Waldman's and Palmer's likely suffer from the fundamentally incorrect assumption that regional differences in the administrative prevalence of autism reflect a real difference in actual prevalence. But I do believe it is possible to use administrative data to draw preliminary conclusions, so long as confounding factors are accounted for.
My intention in writing this post was to walk through an analysis of publicly available data, controlling for population density, to see if the rainfall autism link effect remained. I fully expected there to be a naive ecological association between precipitation and autism. To my surprise, the effect didn't appear to exist in the first place at the US level, and there was no need to control for confounding.
The following is a scatter graph of annual precipitation by state (1971-2000) vs. the 3-5 IDEA prevalence of autism (estimated for 2006).
There's not even an autism trend in the expected direction. This is quite the head-scratcher, and it left me wondering what was going on. Why is it unexpected? Let's first look at a population density map of the US.
It would be reasonable to expect that counties with a higher concentration of people will have higher rates of autism diagnoses, due to increased awareness and a greater availability of autism specialists. Let's now look at a map of precipitation in the US.
The correlation between precipitation and population density is quite clear, isn't it? Why didn't we see an association trend in the expected direction in the scatter graph then? First, it seems that a few states bring the slope down. These would be states with a low autism prevalence but high precipitation rates, like Louisiana, Alabama and Mississippi.
That's a bit of bad luck for Waldman et al. Additionally, we don't have that many data points. There's unfortunately too much variability in this US-level data, which makes it pretty inadequate. Perhaps using 6-11 IDEA prevalence would be better than 3-5 prevalence. In any case, it's doubtful statistical significance would be achieved, and even if it were, it is doubtful it could withstand controlling for population density.
I think the association needs to be revisited in a different way. But this exercise left me wondering why Waldman et al. decided to only look at counties from certain states, namely, California, Oregon and Washington (with California not showing a clear association).
I am going to suggest cherry-picking might have occurred when it comes to Oregon and Washington. In order to argue this point, I will simply post population density and precipitation maps of each of these states. You will see that the pattern in these two states is fairly unique. Most people live in the west side of the state, and that's also where it rains.
To summarize: (1) It was not easy to confirm the reported autism rainfall association. (2) Analysis of any such associations between autism and precipitation should account for population density. (3) Cherry-picking might have occurred in this particular case.