group-difference-test

Check whether two groups, or two points in time, differ by more than chance in data you already have. Always use this skill when someone asks whether region A really differs from region B, whether one customer segment (such as app users and web users) spends more than another, whether an average or rate changed before versus after a change, whether a new process, supplier, branch or cohort performs differently, whether the spread differs between groups, whether a statistically significant difference is big enough to matter, or which test to use, even when data is attached and the question sounds like a quick calculation. Also use it for paired versus independent samples, confidence intervals for a difference, small or skewed samples, and comparing three or more groups. Not for designing or reading a randomised A/B test, not for deciding whether the difference was caused by the group, and not for estimating a single average or rate.