A/B Test Significance & Sample Size
Two-proportion z-test with confidence interval, plus the sample size needed to detect a given lift.
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Method
Two-sided two-proportion z-test with a pooled standard error; the confidence interval for the difference uses the unpooled standard error. Sample size uses the normal approximation for two proportions.
Decide the sample size before you start and do not stop early when the result first looks significant — “peeking” inflates false positives. Run tests for at least one full week to cover weekday patterns.
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