Five men apply for the positions in History and one is hired, and eight women apply and two are hired. The success rate for men is twenty percent, and the success rate for women is twenty-five percent. The History Department has favoured women over men. In the Geography Department eight men apply and six are hired, and five women apply and four are hired. The success rate for men is seventy-five percent and for women it is eighty percent. The Geography Department has favoured women over men. Yet across the University as a whole 13 men and 13 women applied for jobs, and 7 men and 6 women were hired. The success rate for male applicants is greater than the success rate for female applicants.

...
The key to this puzzling example lies in the fact that more women are applying for jobs that are harder to get. It is harder to make your way into History than into Geography. (To get into Geography you just have to be born; to get into History you have to do something memorable.)

-- Simpson Paradox: http://plato.stanford.edu/entries/paradox-simpson/