Your scenario
Approximate normal two-proportion sample size, equal 50/50 split, two-sided 5% significance and 80% power. Relative lift: a 5% baseline with 20% lift becomes 6%, not 25%. Days = total sample ÷ eligible daily visitors, rounded up. Independent binary outcomes, fixed horizon, no repeated peeking or multiple-test correction.
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Scenario chart
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Data table
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