# Why is usually the acceptable probability of type 1 and type 2 errors different?

This question is raised by my supervisor and I don't know how to explain.

Usually the accepted confident level is 0.95 which means that the probability of type 1 error is 5%. But the usually accepted power is 0.8 (Cohen, 1988) which means that the probability of type 2 error is 20%. Why we can accept higher probability of type 2 error than type 1 error? Is there any statistical reason behind that?

He also asked the physical meaning of power = 0.8 (why it is selected as a criteria) which I also have no idea to explain it.

And when we use power analysis to design the experiment we may select effective size 0.3, 0.5 or 0.8 to represent the small, medium and large effects. And my supervisor asked why these numbers are selected. My understanding is that these numbers are suggested based on experience. He immediately asked me what is the experience. I am really frustrating about such questions. My major is not statistics and I need to spend a lot of time on such questions, which I think may not be meaningful. Can any one suggest if such questions are really meaningful or not? If yes, how shall I find out the answer.

• Neither the 5% type one error rate nor the 80% figure for power are universal. Indeed, I doubt your average physicist has even heard of Cohen. Sep 18, 2014 at 10:05
• A jaded, cynical, but likely partially correct answer is "that's what reviewers in your field will demand."
– whuber
Sep 18, 2014 at 17:01