I fit a GEE model looking at the relationship between cognitive score and head impacts. Head impacts is a categorical variable separated into 4 quartiles by amount of exposure.

Here are the results:

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So there does not seem to be an interaction effect overall between neck strength and head impact quartile (last row). However, the third quartile interaction is significant and fourth quartile interaction gets close (3rd and 2nd row from bottom respectively).

Given these results, is it appropriate to conclude that there could be an interaction in the higher head impact quartiles but not in the lower quartiles despite seeing no significant effect overall?

  • $\begingroup$ Did you make the quartiles or were they given to you? (That is, do you have the original continuous measure?) $\endgroup$ – Peter Flom Nov 11 '18 at 11:06
  • $\begingroup$ @PeterFlom Yes I have the original continuous measure. However, given that the number of head impacts is self-reported, I think categorizing into quartiles makes sense and better accounts for the uncertainty. $\endgroup$ – hlinee Nov 11 '18 at 13:06
  • $\begingroup$ I disagree. Quartiles increase uncertainty; if there is already uncertainty in the self report, there will more uncertainty once it is categorized, because it could easily go to the wrong category. $\endgroup$ – Peter Flom Nov 11 '18 at 19:14
  • $\begingroup$ Hmm ok I see. Another reason for categorizing into quartiles is that the data is very right skewed. Either way though, if the goal is to detect a significant interaction effect between neck strength and # of head impacts, does it matter if the variable is continuous or categorical? $\endgroup$ – hlinee Nov 12 '18 at 1:12

First, as Peter mentioned in the comments, it is not a good idea to categorize a continuous predictor. You lose information by doing so.

Second, regarding your specific question, given that the omnibus test for the interaction is not significant, you should not look at the p-values for the individual categories because these are not automatically corrected for multiple testing.

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  • $\begingroup$ So what if I did correct for multiple testing, and some of the individual categories were significant but the overall was still not? $\endgroup$ – hlinee Nov 13 '18 at 23:49
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    $\begingroup$ Typically the omnibus test has a higher power because it more appropriately accounts for the relations between the different categories rather than the multiple-testing-corrected individual tests. $\endgroup$ – Dimitris Rizopoulos Nov 14 '18 at 8:30
  • $\begingroup$ Ok thank you that makes sense. In regards to categorizing a continuous predictor though, does my argument for the data being strongly right skewed be a good enough reason? Or would transformation be a better option? $\endgroup$ – hlinee Nov 17 '18 at 16:37

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