A/B test significance calculator

The proportion test is the pooled two-proportion z-test with an unpooled confidence interval; the means test is Welch's t-test, which does not assume equal variances. Both are fixed-horizon readings: decide at the planned end, because stopping early on a promising p-value inflates the false-positive rate. For tiny conversion counts the normal approximation gets shaky, and the tool says so instead of guessing. Power is a normal approximation against the observed standard error, not a noncentral-t computation.