Evan's Awesome A/B Tools (home):

Sample Size Calculator  |  Chi-Squared Test  |  Two-Sample T-Test  |  Poisson Means  |  Survival Means

Question: How many subjects are needed for an A/B test?

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Baseline conversion rate: %
Minimum Detectable Effect: %
10.2% – 13.2%
The MDE is the smallest difference that can be detected with the prescribed number of subjects.
Conversion rates in the gray area will not be distinguishable from the baseline.
per branch
Statistical power 1−β:
Percent of the time the minimum effect size will be detected, assuming it exists
Significance level α:
Percent of the time a difference will be detected, assuming one does NOT exist
If you like this, check out Wizard — the easy Mac statistics app.

See also: How Not To Run an A/B Test