Post hoc ergo propter hoc

February 4, 2010

Now that The Lancet has retracted the original paper linking autism to MMR it’s interesting to read the comments associated with articles discussing the controversy on both sides of the Atlantic. There are a large number of comments that are all roughly along the lines of “my child had the MMR and was then diagnosed with autism and I know other people like this as well”. Fiona Phillips is, once again, getting in on the act, describing on Jeremy Vine’s Radio 2 program the deluge of correspondence she’s received from parents “without a voice”. She goes on to say that on the basis of so many parents experiences that there must be some link but then quickly states that she’s not against the MMR (confused on where she stands yet?). Fiona does have a history of saying that when it comes to a link between MMR and autism you can’t deny an anecdote!

There seems to be a widespread, basic lack of understanding of the difference between correlation and causation. If A happens and then B happens it doesn’t mean that A caused B. Worse, once correlation has been confused with causation, is the assumption that avoiding “the cause” will change the outcome. If A is avoided then B won’t happen.

I can understand how parents faced with a diagnosis of autism would want to understand why it had happened, to look to blame something or someone for it. However, the overwhelming evidence is that it is nothing more than coincidental timing that autism is generally diagnosed around the same time a child is given MMR.

For the record, both my kids were given the MMR jab.


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