Anscombe's Quartet

Anscombe's quartet is a famous set of four small datasets, built in 1973 by statistician Francis Anscombe, that share almost identical summary statistics — same mean, same variance, same correlation, even the same fitted regression line — yet look completely different when you plot them. One is a clean straight line, one is a curve, one is a perfect line ruined by a single outlier, and one is a vertical stack moved by one stray point.

Its whole purpose is to make one point unforgettable: always plot your data before trusting the numbers. Summary statistics compress a dataset down to a few values, and that compression can throw away exactly the structure — curvature, outliers, clusters — that you most need to see. The numbers agree; the pictures scream that the four are nothing alike.