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This started as a simple discussion where everyone thought they new the answer, and ended up with arguments and dozens of paper quotations. Thoughts appreciated.

We have 10 samples: Control, A, B, C, ..., I, with good enough sample size, and the groups have similar variances.

The following comparisons were made using unpaired two-sided t-tests:

A - Control

B - Control

C - Control

...

I - Control

We obtained 9 p-values. Do we have to adjust these p-values to account for family-wise issues ?

Thanks!

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  • $\begingroup$ Have you noticed that all nine comparisons are correlated? (This is because they all use the same control.) $\endgroup$
    – whuber
    Nov 17, 2014 at 18:00

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Yes, it is necessary to adjust for repeated testing to control for increasing probability of false positives. In terms of reporting, I think it is best to report both raw and adjusted p-values, specifying which correction was used (eg Bonferroni). But why did you do a series of t-tests instead of eg ANOVA with post-hoc comparisons?

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    $\begingroup$ In the same fashion, why not just code the samples as a categorical variable, and run a regression on it, which would yield straightforward p-values per parameter? You compare them to the same reference category anyway. $\endgroup$
    – Maxim.K
    Nov 17, 2014 at 18:22
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You have multiple treatment groups that are each compared to the control group, but not compared to each other. There's a method called the Dunnett procedure designed specifically for controlling the familywise error rate in this situation.

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