By validation, I mean how do we, as instructors or practitioners, know that what we are teaching or practicing will actually work if it is needed in a real situation.
I think it's a trickier question than most people realize. I could say "hey, I was in this situation that could have turned violent and I talked my way out of it, so my de-escalation skills work." Maybe so, but maybe lots of other people without any training would have done just a good a job of it. Maybe some of them could have done better. Maybe the person I was talking down was just blustering and didn't really want to fight anyway. I could say "hey, my student successfully defended himself against a mugger, so the techniques I teach work." Maybe, but I teach lots of techniques. What about the ones my student didn't use? Do I know those work? Do I know that the techniques which worked for this student would have worked for another student? Do I know that the mugger wasn't just a wimp? I could say "these techniques were passed down from warriors who used them in life-or-death combat, so they must be effective." Maybe so, but do I have reliable historical records of exactly who used the techniques in what circumstances against whom and what their rate of success was? Probably not. For that matter, suppose a founder of an art added a technique to the system because it saved his life in battle. Suppose the next ten people from that system who tried the same technique in combat failed miserably and died as a result. Were their ghosts able to report back to the head instructor and say "hey this technique sucks, drop it from the curriculum please?"
I think that as martial artists we've invested a lot in our training and we want the certainty of knowing "this is the real stuff. This will keep me (or my students) safe." In real life, it's hard to really know that for sure, except within certain limited contexts. The rest of the time we have to do the best we can with limited knowledge and accept
a certain degree of uncertainty.