Make decisions with data — the science behind every Booking.com, Airbnb, Netflix, Google & Meta A/B test. Experimental design, control vs treatment, randomization, hypothesis testing (null & alternative), significance level α, p-values, confidence intervals, power analysis & sample-size calculation (G*Power, Evan Miller calculators), MDE (minimum detectable effect), one-sided vs two-sided tests, t-tests / z-tests / chi-square / Mann-Whitney / bootstrap, multiple-comparisons (Bonferroni, FDR, Benjamini-Hochberg), CUPED variance reduction, sequential testing & the peeking problem, novelty & primacy effects, SRM (sample ratio mismatch), interaction & carryover effects, multi-armed bandits, Thompson sampling, switchback experiments, geo experiments, holdouts & long-term metrics, OEC (overall evaluation criterion), guardrails, network & spillover effects, segment analysis, Simpson's paradox, ethics & informed consent, tools (Optimizely, VWO, LaunchDarkly, GrowthBook, Statsig, Eppo), and the famous "Twyman's Law". 30 units, 450 lessons.