How to Calculate Statistical Power
What is Statistical Power?
Statistical power (1−β) is the probability of correctly detecting a true effect. Power of 0.80 (80%) is the conventional minimum — a 20% chance of missing a real effect (Type II error).
Step-by-Step Guide
- 1Power increases with: larger n, larger effect size, higher α
- 2Plan sample size to achieve 80% power before collecting data
- 3Power = Φ(|d|√(n/2) − z_α)
Worked Examples
Input
d=0.5 · n=50/group · α=0.05
Result
Power ≈ 70% — underpowered
Need ~64/group for 80% power
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