Evan Miller's “Statistical Formulas For Programmers”
Being able to apply statistics is like having a secret superpower. […] Unfortunately, not enough programmers have this superpower. That’s a shame, because the application of statistics can almost always enhance the display and interpretation of data.
As my modest contribution to developer-kind, I’ve collected together the statistical formulas that I find to be most useful; this page presents them all in one place, a sort of statistical cheat-sheet for the practicing programmer.