Simpson's Paradox Reverses Trends

The scariest statistical illusion is Simpson's paradox — a trend that appears in every subgroup but reverses when the groups are combined. In the 1973 UC Berkeley case, the university's overall acceptance rate was higher for men, suggesting bias. But broken down by department, women were accepted at the same rate or higher. The overall figure misled because men and women applied to different departments.

The paradox shows up in medical trials, sports rankings, and economic indicators. Never average when you can disaggregate — look at the subgroups before drawing a trend line.