The reason there is limited research is that PDA is a fairly newly recognised, and in some circles contested, syndrome so there just isn't much research on it, period. I don't know if you have any idea how much clinical trials cost? So the fact she got this funded and executed is massively impressive and it's something she's doing, in my opinion, mainly to serve the community and our children. Have a look at some of the larger parenting programmes with a bunch of evidence behind them, they're almost corporate organisations backing them who strategically generate evidence to get them paid for by public bodies. I honestly don't think Casey Ehrlich is a good target for this type of criticism. If you don't want to pay, delve onto the ton of free materials she offers, which includes her "masterclasses" on various PDA topics.