80% power (1-β=0.8) is the standard target — meaning 20% chance of missing a real effect.
For d=0.5 effect, α=0.05, 80% power: need n=64 per group in a two-sample t-test.
This calculator is appropriate for any analysis that will be presented, reported, or used to make decisions. Accurate statistical metrics are the foundation of valid conclusions.
Many users interpret statistical results without considering the distribution of the data. Standard deviation is most meaningful for roughly normally distributed data — in highly skewed datasets, the median and IQR are more appropriate measures of centre and spread.
A teacher calculates the class average and standard deviation for an exam: mean 64%, SD 18%. The high SD suggests a wide spread of performance levels — indicating the class may need differentiated support rather than a single revision strategy.
For a medium effect (d=0.5), α=0.05, two-tailed t-test: ~64 per group (128 total).
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