Mathematics
Nonparametric Tests: Sign Test and Rank-Based Methods
Quick fact
The sign test is one of the oldest statistical tests—it was used by John Arbuthnot in 1710 to argue that more boys than girls are born each year, showing a divine providence. It compares counts of positive and negative signs without needing any distributional assumption.
Why this is interesting
What if your data is skewed, full of outliers, or not even numeric? The trusty t-test falls apart—but there's a whole class of tests that don't care about the shape of your data.