1. You have to have a randomized study to balance biases and other confounders. Otherwise, there are too many things that differ from family to family and state to state, etc to make a logical conclusion between a child's developmental changes at 18 months and the shots they receive at 2,4,6 months.

2. You'd need huge numbers to rule out chance. Huge.

3. You'd likely need to recruit heavily from the Anti-vax crowd. Good luck.

4. There aren't any real alternatives being suggested, other than the Portland guy, which might actually be a reasonable study, but you can't do it with "Home schoolers" like it was implied. That's a huge confounder.

5. Risk to the kids. You'd have to have a much better scientific basis to your claims than "my kid got his shots at 12 months and then got worse" That's like saying high school attendance is the cause of teenage pregnancy since kids suddenly start getting pregnant a lot more once they attend high school. This is a big issue. Many of these diseases are real threats to children. We are talking about brain damage, hearing loss, vision loss, developmental delay, and death. We know that these diseases cause these side effects, and not just a little bit. Talk to someone in their 70s about growing up pre-Polio vaccine. It is real. I've met people with post-polio syndrome and it doesn't go away. You will need a really good reason to expose 10k-20k kids to these diseases and because of an equal amount of intellectual dishonesty on the anti-vax side as there is on the vax side, it isn't likely.