What percentage of the time does the average woman fake an orgasm, and why?

Like most guys, I think I'm pretty good at knowing when my partner is faking an orgasm and when it is real. However, I'm not 100% sure every time. It actually doesn't bother me either, at some point there needs to be "closure". It's not like you can just stop and go to sleep.

On the flip side, she knows for sure, since it's pretty obvious. So, what percentage of the time does the average woman fake an orgasm, and why?