Benford's Law

In a big pile of real-world numbers, how often does each one start with a 1?

You'd probably guess about one time in nine. There are nine digits it could be, so why would any single one win out? But look at a real pile of numbers, like river lengths or country populations, and the first digit is a 1 far more often than anything else.

It's such a reliable habit that fraud investigators use it to catch cooked books: invented numbers almost never follow it. Have a look for yourself.