Underpowered tests, peeking at results early, and ignoring novelty effects produce false wins that evaporate in production. A result that looks like a 9% lift on day two often regresses to noise by day fourteen. Significance is not the same as a durable effect.
Decide your sample size and duration before you launch. Watch the confidence interval, not just the point estimate. Always ask whether the lift survives a full business cycle: weekday and weekend, paid and organic. The discipline of not fooling yourself is the actual skill.
