In a real Scientific Study it is RANDOMIZED which subjects get MMR and which do not. When parents select which children do not get MMR, it is the children who are showing signs of Autism who do not get MMR
As mentioned numerous times, you can’t choose random children to not vaccinate. It’s unethical.
You can’t have it both ways. You demand retrospective studies, then complain about the inherent flaws in a retrospective study.
These are scientific studies made by professionals. I’m sure they control for these biases as best as possible, and, for example, exclude autistic children who didn’t get vaccinated because there were already signs of autism before they were to receive their immunization.