I ran a logistic regression model with PTSD, MDD, Nativity, and (PTSD*Born outside the US) interaction term predicting Nicotine Dependence (yes/no). The main effect of Born Outside the US (ref: born in the US) has OR=9.13, PTSD main effect OR=2.12. However, the interaction term of PTSD and Born outside the US has OR=0.31. I find it very strange that the OR changed direction. Can anyone advise on the potential explanations for such results?
Hi Stanislava Klymova, interesting questions!
May I ask you how the OR changed before/after you include the interaction term?
Was the p-value of the OR of the interaction term significant?
Hi Stanislava Klymova, by including this interaction term in your model, you're assuming that the effect of being born outside the US is different for those with/without PTSD. Another way of interpreting this coefficient is the 'extra effect' of being born outside the US while having PTSD (assuming you included these predictors as binary variables with yes = 1). It may be a good idea to first run a likelihood ratio test comparing two models - one with/one without the (PTSD*Born outside the US) interaction term to test whether including this effect modification improves the fit of your model.
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