Statistical Power Calculator
Calculate the power of a proportion test and the effect it can detect.
About Statistical Power Calculator
Power depends on sample size, baseline rate and the true effect. Comparing the observed lift with the minimum detectable effect shows immediately whether the test could ever have found it. A test whose minimum detectable effect exceeds any plausible business improvement is not worth running at all.
Formula
Statistical Power
Power derives from the z of the observed difference less the critical value
- minimum detectable effect
- Smallest lift detectable at 80% power with this sample
Worked example
With the default values below, Statistical Power Calculator returns Statistical Power of 41.959%. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Sample size per groupminimum 2
- 5,000
- Baseline rateaccepts 0.01–99 %
- 5 %
- Observed rate in the variantaccepts 0.01–99 %
- 5.8 %
- Significance levelaccepts 0.1–20 %
- 5 %
Results
- Statistical Power
- 41.959%
- Minimum Detectable Effect at 80 Per Cent Power
- 1.2214%
- Observed Absolute Lift
- 0.8000%
- Observed Relative Lift
- 16.000%
- Verdict
- Underpowered — this effect size may not reach significance
Step by step
- 1Values used
- 2Statistical Power
- 3Statistical Power= 41.959
- 4Minimum Detectable Effect at 80 Per Cent Power= 1.2214
- 5Observed Absolute Lift= 0.8000
- 6Observed Relative Lift= 16.000
- 7Verdict= Underpowered — this effect size may not reach significance
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Statistical Power calculated?
Power derives from the z of the observed difference less the critical value Power depends on sample size, baseline rate and the true effect. Comparing the observed lift with the minimum detectable effect shows immediately whether the test could ever have found it.
Why does Statistical Power matter?
A test whose minimum detectable effect exceeds any plausible business improvement is not worth running at all.
What values do I need to enter?
This calculator takes 4 inputs: Sample size per group, Baseline rate, Observed rate in the variant, Significance level. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.