Which expression defines statistical power?

• A. Beta
• B. The probability of a Type I error
• C. The probability of rejecting the null when it is true
• D. 1 minus beta

Answer: (D)

Power is the probability that a study will correctly reject the null hypothesis when the alternative is true. It is equal to 1 minus beta, where beta is the probability of a Type II error (failing to reject a false null). This is not the probability of a Type I error, which is alpha, the chance of incorrectly rejecting a true null. It’s also not the probability of rejecting the null when it is true. Power increases with larger sample size, larger true effect size, and lower variability, and it can be influenced by the chosen significance level (alpha). In practice, researchers often aim for a power around 0.8. Therefore, the expression for statistical power is 1 minus beta.