I am running a cox proportional hazards regression with Stata and am having trouble interpreting the interaction term between two binary categorical variables.
For simplicity, here's a summary of the output and the problem i'm struggling with:
stcox a##b gives me the following:
Hazard Ratio for A: 1.1 (p <0.01) Hazard Ratio for B: 1.1 (p=0.3) Hazard Ratio for A#B: 1.4 (p<0.01).
Now, my question is the following:
Do I interpret the Hazard Ratio for the interaction term as
-the interaction between A and B provides a hazard ratio of 1.4
-the interaction between A and B provides a hazard ratio that is 1.4 times greater than what would be expected by the sum of A and B if there was no true interaction present
The reason why I ask is because when I just include the interaction term without the main effects (stcox a#b), I get a hazard ratio of 1.80.
Thank you so much!