Chapter 2 ended with the +$29 discount-coupon effect and one open question: how do we know that number isn't just noise from one particular two-week window? Answering it needs exactly one formula — simple enough to compute by hand, and dangerous enough, when ignored, to have earned its own nickname in the statistics literature.
The Secret Behind Your Top-Converting Product
Suppose you rank every product by its purchase conversion rate — purchases divided by views — and look at the top 1%...