Free Power Analysis Calculator
Estimate the required sample size per group to achieve a desired statistical power for a two-sample z-test.
Sample Size per Group
31.40
Total Sample Size (2 groups)62.81
Target Power (%)80
Sample Size per Group vs Effect Size (Cohen's d)
How to Perform a Power Analysis
Formula
n per group = ((z_alpha/2 + z_beta) / d)^2
Power is the probability of correctly rejecting a false null hypothesis (detecting a real effect). This formula estimates the sample size needed per group for a two-sample test. Higher power, smaller effect sizes, and lower alpha all require larger samples.
Example Calculation
Detect a medium effect (d = 0.5) with 80% power at alpha = 0.05.
- 01n = ((1.96 + 0.842) / 0.5)^2
- 02= (2.802 / 0.5)^2
- 03= 5.604^2
- 04= 31.4
- 05Round up: 32 per group, 64 total
Frequently Asked Questions
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