How to know what actually works

You changed something and the numbers moved, but did your change cause it? Answering honestly is harder than it looks, and getting it wrong wastes fortunes on things that never worked. Learn why before-and-after comparisons fool you, how a randomized experiment reconstructs the missing counterfactual, the statistical traps that make even a clean test lie, and why experiments quietly break on the social and marketplace platforms where they matter most.