Re: Main and interaction effect in Cox Prop Hazards

Posted by Bruce Weaver on Sep 20, 2019; 10:02pm
URL: http://spssx-discussion.165.s1.nabble.com/Re-Main-and-interaction-effect-in-Cox-Prop-Hazards-tp5738390p5738394.html

Rich Ulrich wrote
> I'm not sure what Bruce was saying about coefficients.

I'll try again.  Here are the B values for the two variables of interest and
their interaction:

B(drink) = 0.566 = ln(HR) for drink when ses = 0 (i.e., reference category)
B(ses) = 0.393 = ln(HR) for ses when drink = 0 (i.e., reference category)

B(interaction) = -0.094

Therefore:
0.566 -0.094 = 0.472 = ln(HR) for drink when ses = 1
0.393 -0.094 = 0.299 = ln(HR) for ses when drink = 1

The interaction term tests the differences between 0.566 and 0.472 (for
drink) and between 0.393 and 0.299 (for ses).  

And of course, those ln(HR) values are adjusted for the other variables in
the model.  

Is this clearer?  




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Bruce Weaver
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