I don't know of any scientific studies which the best type of broom to sweep a hallway with or the best hammer to hit a nail into a piece of wood with. Yet somehow I've managed to use these tool.

I don't know of scientific studies showing the effectiveness of repeated affirmations (though I'd suspect there are some). They still seem like a very plausible tool.

Extraordinary claims require extraordinary evidence. By that token, there are some claims so ordinary that they only require the ordinary evidence of experience.