Power and sample size
How many replicates you need — worked out before the experiment, not after.
Runs in your browser
The smallest difference worth acting on.
%
Per group
128 in total
Gives 80.1% power. One fewer drops it to 79.5%.
Formula
power = P(|T′| > t₍₁−α/2₎), T′ noncentral t with δ = d√(n/2)ModelNoncentral t, two-sided
- Cumulative distribution function of the noncentral t — Applied Statistics, 1989
- Conventional small, medium and large effect sizes — Statistical Power Analysis for the Behavioral Sciences, 1988
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Do this before the experiment. Power computed after a non-significant result, from the effect you happened to observe, is circular and tells you nothing.
About this tool
Calculates the sample size required to detect an effect, or the power you have at a given n. Uses the noncentral t distribution rather than a normal approximation, which matters most at the small sample sizes bench experiments actually run at.
Related tools
- t-test calculator — Compare two groups and get the effect size and interval, not just a p-value.
- One-way ANOVA — Compare three or more groups, with variance explained and every pair tested.
Science last reviewed .