Wolfram Word Frequncy

[akoaysigod]

I was looking up a definition on wolframs earlier today for a word and saw that they have a word usage frequency for the past 500 years. I'm assuming they determine this using some sort of statistics/math. I tried researching it but I've just run into dead ends. I know there are some people here who are really good at math and I'm wondering if anyone could point me in the right direction into how they're determing this.

I really don't like statistics and I've not really taken any courses explicitly in it but I would like to know the approach they're taking in order to calculate this sort of stuff.