What are your samples of?
Samples of populations, analyzed for certain traits. So long as we're able to conclude that the distribution of the sample has a 5% chance or better of being a random deviation from the "expected" distribution of the population, we conclude that it is.
Like, if we have a sample of people and we're looking at their height, and we find that there's a 5% chance or greater that the difference between the distribution in our sample and the distribution in the whole is due to nothing more than chance, then we conclude that the sample we have is truly random, and not the result of some kind of selection for heights.
Again, unless I'm way off base, here, we take anything over 5% confidence. Yeah, I was surprised, too.