Thursday, November 15, 2007

Clinic Notes: The Autism-Vaccine Debate: Stat 101. Correlation and Causality

Autism takes two forms: Infantile, which is present form infancy, and regressive where the child develops normally and then between 2 and 3 years of age begins to loose language and appropriate social behavior. Often self-stim and sensory problems also put in an appearance at this time. Of course, concerned parents look for a cause and the only thing that they note that was different was the MMR vaccine that there child received before autism began to develop. Further inquiry by the parents finds that the vaccine contained mercury, which is known to be toxic to the brain.
Congressional hearings have been held, research supporting and refuting the mercury-autism link has been presented and mercury has been removed from vaccines, but the debate goes on.
Stat 101. Much of the retrospective research on health issues relies on a statistical test know as correlation. A correlation will tell you if two variables are statistically related. So height and weight are correlated, intelligence and grades are correlated, and cigarette smoking and cancer are correlated. Since vaccinations are mandatory for the most part in the US and everyone receives the MMR vaccine it is hard to come up with an accurate. In parts of Europe where vaccinations are not mandatory the rates of autism are the same in vaccinated and non-vaccinated groups of children. What most people don't understand is that a correlation will tell you if two variables are related, but it will tell you nothing about causality. Just because autism follows the MMR vaccination does not mean that the vaccination caused the autism. Maybe mercury does play a role in autism. Increasingly there is more mercury in our environment as well as other neurotoxins. But a correlation cannot make a statement about causality.