If they had developmental problems beforehand, that doesn’t negate the whole study. It would narrow it down to those with developmental problems.
I once heard an interview on NPR about this. The guest was mocking the whole idea, until the host interjected. He said, “I won’t let you get away with this one. My daughter became autistic shortly after having the vaccine.” He really stumped the interviewee, who wasn’t expecting anything like that from the, usually impartial, host.
It seems to me that there might be a lot more “anecdotal” evidence that is being ignored by those who didn’t observe it, but is being circulated
amongst friends.
The bottom line is that everyone here has an ax to grind.