Stratified Sampling

The 96% Turned Red the Moment We Split It by Channel — Two Fatal Failures of Eval Sampling

The vendor drew 200 rows at random and reported "96% accuracy." I split those 200 by channel — in the small public-domain channel, the price-difference cases were nearly all wrong, and the high-traffic private channel had diluted it into a pretty number. In 5 minutes you'll see through both failures of random sampling (missing the tail + a big channel drowning a small one); in 20 you'll have a stratified frame with channel drill-down and risk-layer over-sampling.