What is the purpose of a power analysis?

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Power analysis is used to plan how large a study needs to be to have a good chance of detecting a real effect if it exists. It quantifies the study’s power—the probability of rejecting the null hypothesis when the true effect is at least a specified size—given a chosen significance level. By specifying the smallest effect worth detecting and a target power (often 80% or 90%), you can determine the required sample size. The needed size depends on the expected variability of the outcome and whether the test is one-tailed or two-tailed. This planning helps ensure the study isn’t underpowered or unnecessarily large. The other options describe different tasks: estimating variability, adjusting p-values after data collection, or choosing a statistical test. So the purpose is to determine the sample size needed to detect a given effect with a specified probability.