Re: Main and interaction effect in Cox Prop Hazards

Posted by Rich Ulrich on Sep 21, 2019; 9:47pm
URL: http://spssx-discussion.165.s1.nabble.com/Re-Main-and-interaction-effect-in-Cox-Prop-Hazards-tp5738390p5738397.html

Bruce,

I hate it when interactions have to be interpreted, especially if it

is possible that the program generated them and I don't have control.

Also, telling a program that this is a "factor" and not a continuous

"variable" raises the problem that, for 0/1,  if the program takes "1" as

the reference group, it reverses the sign of the B obtained. PITA.

Further:  if two variables are coded as 0/1  and the interaction is their

simple product - instead of the "centered" product - then the interaction's

B is correlated with the main-effects' B's, and affects their magnitude in

the equation that has them all.

I recognize that this OP needs basic help with interpretation of the model,

which you provided.  I was skipping that, because the main thing relevant

to the question is that there /is/  no interaction effect worth worrying

about, according the Wald test.

--

Rich Ulrich

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From: SPSSX(r) Discussion <[hidden email]> on behalf of Bruce Weaver <[hidden email]>
Sent: Friday, September 20, 2019 6:02 PM
To: [hidden email] <[hidden email]>
Subject: Re: Main and interaction effect in Cox Prop Hazards

 

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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