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Statistical Confidence For Test Calculator

Work out statistical confidence for test instantly with clear inputs, formula shown and shareable results.

About Statistical Confidence For Test Calculator

A two-proportion z-test compares conversion rates using the pooled standard error. The confidence level is the probability that a difference this large would not arise by chance if the variants were truly identical.

Formula

Two-proportion z-test

z = (p_b - p_a) / √[p̄(1-p̄)(1/n_a + 1/n_b)]

Worked example

With the default values below, Statistical Confidence For Test Calculator returns Statistical Confidence of 98.856%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Visitors In Controlaccepts 1–100,000,000
12,000
Conversions In Controlaccepts 0–100,000,000
384
Visitors In Variantaccepts 1–100,000,000
12,000
Conversions In Variantaccepts 0–100,000,000
456

Results

Statistical Confidence
98.856%
Z-Score
2.5289
Relative Lift Of The Variant
18.750%
Conclusion
Statistically significant at 95%

Step by step

  1. 1Values used
  2. 2Two-proportion z-test
  3. 3Statistical Confidence= 98.856
  4. 4Z-Score= 2.5289
  5. 5Relative Lift Of The Variant= 18.750
  6. 6Conclusion= Statistically significant at 95%

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.
  • Figures are estimates based on the inputs given. Marketing performance, platform fees and conversion behaviour vary by audience, channel and season. Use this as a planning guide, not a forecast.
  • This is an estimate for general information and not professional financial, tax or legal advice. See the full disclaimer for the limits that apply to this kind of calculator.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Figures are estimates based on the inputs given. Marketing performance, platform fees and conversion behaviour vary by audience, channel and season. Use this as a planning guide, not a forecast.

Frequently Asked Questions

Is 95% confidence a guarantee?

No. It means a one in twenty chance of a false positive, which is why peeking at results repeatedly inflates errors.

Why does the lift matter as well as significance?

A statistically significant 0.2% lift may not be worth implementing. Significance and materiality are different questions.

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