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:
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?