Not an assumption. Small sample, no question about it, but having seen .22's fail to "stop" people, and having seen people shot with .38's who went down right away, I do have more faith in .38's. I sent a message to DocGKR, who might be better able to address the issue of terminal ballistics.
As I said above, what you consider effective, and what I consider luck, seem to be closely related. Effectiveness, for me, relates to just how fast an attacker ceases to be able to continue his attack. If an attacker chooses to leave, that's not good enough. If you attack me, you lose the ability to choose your actions, just as fast as I can make that happen.