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Can you power a retrospective study?
Retrospective studies use statistical power rather than the calculation of sample sizes and we call these ‘post hoc power analyses’.
What is retrospective power?
A retrospective power and sample size analysis is an analysis that you use to determine whether a test that you already performed had enough power.
Why post hoc power analysis is bad?
Post hoc power analysis identifies population-level parameters with sample-specific statistics and makes no conceptual sense. Analytically, such analysis can yield quite different power estimates that are difficult and can be misleading.
Which is a better method to calculate post hoc power?
As an alternative to post-hoc power, analysis of the width and magnitude of the 95% confidence interval (95% CI) may be a more appropriate method of determining statistical power. To calculate an adequate sample size for a future or planned trial, please visit the sample size calculator.
Is it wrong to report post hoc power?
We respectfully disagree that it is wrong to report post hoc power in the surgical literature. We fully understand that P value and post hoc power based on observed effect size are mathematically redundant; however, we would point out that being redundant is not the same as being incorrect. . . .
How is the statistical power of a study calculated?
Just like sample size calculation, statistical power is based on the baseline incidence of an outcome, the population variance, the treatment effect size, alpha, and the sample size of a study.
Which is the best approach to retrospective power?
There are differing approaches to retrospective power, but the one of interest in this article is a power calculation based on the observed value of the effect size, as well as other auxiliary quantities such as the error standard deviation, while the significance level of the test is. held at a specified value.