Tools/Confidence Interval Builder

Confidence Interval Builder

For sample means and proportions, with a built-in sample-size calculator for A/B tests. Pick a target margin of error, get the required N.

Inputs

Type Confidence level

Sample mean x̄ Sample size n Std dev σ (or s)

Result

Visualization

A/B testing without statistical nonsense

The hardest part of A/B testing isn't computing the p-value — it's deciding before the test starts how big a sample you need. Stopping "when significant" inflates false-positive rates to 20%+ (see Simmons, Nelson, & Simonsohn, 2011). Use the sample-size calculator to commit to N up-front.

Who uses it

PMs / growth teams sizing A/B tests before launch.
Survey researchers estimating population proportions.
Students doing stats homework without paying for SPSS.

Limitations

Sample-size formula assumes normal approximation is valid — fine for np ≥ 10 and n(1−p) ≥ 10.
We don't model unequal variance (Welch's t-test) — for that, use the dedicated tool in your stats package.
For sequential A/B testing (peeking), use a different methodology (e.g. mSPRT, always-valid inference).