Quick guide
How to use this calculator
- Enter the observations, probabilities, model parameters, or summary statistics requested by the visible labels.
- Keep every value on the same scale and confirm that the selected sampling relationship, distribution, and tail convention match the question you are investigating.
- Read the result together with its assumptions and interpretation. Statistical output summarizes uncertainty under a model; it does not repair biased data or establish causation.
Calculation method
How the a/b test sample size and power calculator works
Two-proportion normal planning uses pooled critical variance and alternative-group variance.
Report absolute and relative lift together; a statistically detectable change may still be commercially trivial.
Worked example
A/B Test Sample Size and Power example
A baseline of 10% and alternative of 12% requires thousands of visitors per arm for conventional power.
Two-proportion normal planning uses pooled critical variance and alternative-group variance.
Supported inputs
Precision and limits
Model and design
Assumes independent visitors, one observation per unit, fixed analysis, no peeking correction, and stable conversion definitions.
Numerical scope
Inputs use double-precision numerical methods with guarded domains. Datasets accept up to 10,000 finite plain-decimal values. Extremely large parameters or probabilities deep in a numerical tail may require specialist statistical software.
Interpretation
Report absolute and relative lift together; a statistically detectable change may still be commercially trivial.
Decision boundary
The calculator does not validate how data were collected, diagnose dependence or bias, choose a scientifically meaningful effect, or replace review by a qualified statistician for consequential research, medical, regulatory, safety, or policy decisions.
Privacy
Entered values and calculated results stay in this browser and are not sent to an analytics service.
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