doomsday
“Please continue.”
gladly!
Ok on to the study
Theri objective is “To report ASD occurrence by MMR vaccine status in a large sample of US children who have older siblings with and without ASD.”
with me?
So here is what they did.
They did a retrospective cohort study. what this means , is they look back (retrospective) at an exposure (vaccines) and then look to see if the disease (autism) developed. They look at 2 groups one exposed to the risk and one that wasnt .
note these people are already vaccinated , we aren’t randomizing them to the vaccine vs non-vaccine groups. The exposure already occurred long ago, the disease also occured already (as opposed to a prospective cohort study)
Still with me?
It gets just a bit more complicated, because this study has a twist it isnt a straight retrospective cohort study) .
so lets pause and make sure the general design is clear .
At the same time lets keep the definition of “healthy user bias” tucked in the back of our minds “it is a sampling bias: the kind of subjects that voluntarily enroll in a clinical trial and actually follow the experimental regimen are not representative of the general population. They can be expected, on average, to be healthier”
So far so good?