Lies, damn lies,
And statistics. It always helps to have a refresher on statistical fallacies. From Lifehacker.
Four Common Statistical Misconceptions You Should Avoid
The Base Rate Fallacy That Finds Too Many Terrorists
Here's how the base rate fallacy works: say you have a company with 25% female employees and 75% male employees. From the outside, this would appear to be a biased selection of male candidates. We assume this because in the United States, the gender distribution is roughly equal. However, this ignores the pool of applicants. If only 10% of applicants were females, then a higher percentage of women who applied were selected versus the percentage of men who applied.
Another common example involves the mythical terrorist-spotting device. Imagine a box that has a 99% success rate at positively identifying a terrorist and a 99% chance at properly identifying a non-terrorist correctly. One would assume that if—out of population of 1 million people, 100 of whom are terrorists—the box identifies a person as a terrorist, there is a 99% chance it's correct. In reality, it's a lot closer to 1%. The reason being that the box falsely rang for 1% of non-terrorists (9,999 people), as well as correctly ringing for 99% of real terrorists (99 people).