They just didn't have enough evidence, and persuaded themselves that the evidence they had was better quality than it actually was. The surgery isn't outlandish; it has a plausible mechanism of action; patients report good results afterwards. And the reason best standard research wasn't done is because there's a bunch of stuff we do that we've been doing for a long time and are only just now getting around to properly studying.
So, when a placebo-controlled blinded study was done we discover that knee arthroscopy probably doesn't do anything more than placebo for osteoarthritis.
> Carving an exception like that for oneself
Well, I guess it depends on how aware people are of cognitive bias, and how much they're closing themselves off to the possibility that they're subject to biases.
These things are really hard to guard against.
I agree with you and like the example, but I wanted to provide feedback that this example is confusing in its relation to the current topic.
We know that we should only trust double-blind controlled randomised trials. This surgery hadn't had that trial, thus it shouldn't have been trusted, yet it was. And the reasons it was trusted are just biases.