1
$\begingroup$

I'm a novice in research and wondering if I need to calculate power to determine my sample size. My study is about collecting data from all patients that presented to a clinic for a period of 6 months. Since I am collecting data from all the presentations, would this still require power analysis? If so, could someone guide me on how to do a post hoc analysis as I heard this is the most suitable for a retrospective audit. I believe this is done after the data has been collected to check if the sample size is effective. So would my research proposal just include that a psot-hoc analysis will be completed after data collection has been completed..?

Happy to hear any suggestions if a another power analysis option is more suitable.

Thankyou!

$\endgroup$
1
  • $\begingroup$ Why do you ask about post-hoc power when you haven't started the experiment? (I guess this from the phrase "my research proposal".) There may be opportunity to do a-priori power analysis instead. Why have you picked 6 months to collect data? Since longer study means more patients presented at the clinic, you can design (the length of) the study so that it achieves the desired power. $\endgroup$
    – dipetkov
    Oct 21, 2023 at 20:43

1 Answer 1

4
$\begingroup$

Post hoc power analysis is generally not recommended. See e.g. this article which says that they are "completely determined by p-value'' and demonstrates this. If the p value is high, power must be low and if the p value is low, power must be high.

More generally, the purpose of power analysis is to tell you how many subjects you need in your sample to have a good chance of getting a significant result if the null is, in fact, false by a certain amount.

Also see Hoenig & Heisey (2001), The Abuse of Power: The Pervasive Fallacy of Power Calculations for Data Analysis, in The American Statistician, 55, 19--24, and more references cited there.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.