It’s tough to prove gender bias.
In a real-world setting, typically the most we can do is identify differences in outcome. A man is selected for hire over a woman; fewer women reach tenure track positions; there’s a gender gap in publications. Bias may be suspected in some cases, but the difficulty in using outcomes to prove it is that the differences could be due to many potential factors. We can speculate (...) - By Ilana Yurkiewicz, Unofficial Prognosis blog (Scientific American Blog Network), September 23, 2012