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I have an experiment in which I'm doing, among other things, a One way Anova with one dependent variable and 5 groups. After the ANOVA i'm doing Tukey's post hoc to test for differences between groups.

I want to conduct a power analysis to determine if my power is enough for detecting differences between groups.

I tried g power but it only gives you the power for the omnibus test.

Any ideas on how to do it? Would I end up having a power value for each of the comparissons?

Thanks in advance

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  • $\begingroup$ Hint: think in terms of the minimum difference necessary. With post-hoc tests you will I think be testing whether any single group difference is at least as large as ___, and you'll assume all other differences will be smaller than that. $\endgroup$ – rolando2 May 8 '18 at 20:06
  • $\begingroup$ If you are planning and conducting the experiment, you probably have hypotheses in mind. Therefore, you could/should use confirmatory, a-priori contrasts (aka planned-comparisons-test) instead of rather exploratory post-hoc comparisons. And thus you can conduct power analysis for contrasts. $\endgroup$ – hplieninger May 9 '18 at 7:57
  • $\begingroup$ Thank you @rolando2 . I'm not sure if I undestood correctly. Are you saying that I should grab the minnimum difference between 2 groups, do the power analysis for that single t test and then if I have enough power to detect that one I will have powe to detect the other (larger) ones? If that is the case. How to take into account the correction that is made by Tukey post-hoc? $\endgroup$ – Gonzalo Lerner May 10 '18 at 14:18
  • $\begingroup$ Thanks @hplieninger i found some info on power analysis of contrasts. In that case I wouldn't correct for multiple comparissons? What if I'm actually interested in the difference between all groups? $\endgroup$ – Gonzalo Lerner May 10 '18 at 14:45
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    $\begingroup$ You don't need a correction for contrasts. With five groups, you can test four specific hypotheses; do you really have more? Then you're left with post-hoc tests (with correction). $\endgroup$ – hplieninger May 11 '18 at 7:10

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