Plot It Before You Conclude: What a Table of Numbers Hides

Summary statistics are a compression, and like any compression they throw information away. A mean, a spread and a trend line can be identical for data that behaves in completely different ways — and the difference is obvious the moment you draw it. This is not a hypothetical worry. Frank Anscombe published four small datasets in 1973 that share their mean, their variance, their correlation and their fitted line to two decimal places, and look nothing whatever like each other on paper. So the rule is: plot it FIRST, before computing anything. The plot takes a minute, it cannot be fooled the way a summary can, and it is the only step that reliably catches a single bad point, a curve you assumed was a line, or a group that should never have been averaged together.