Re: Main and interaction effect in Cox Prop Hazards

Posted by Bruce Weaver on Sep 20, 2019; 6:28pm
URL: http://spssx-discussion.165.s1.nabble.com/Re-Main-and-interaction-effect-in-Cox-Prop-Hazards-tp5738390p5738392.html

In Nabble, at least, your table was unreadable.  But by coping it, pasting
into Word, and saving as a PDF, I was able to make a readable version.
There should be a link to it below.  (Members who do not use Nabble may need
to view this thread in Nabble to get access to it:
http://spssx-discussion.1045642.n5.nabble.com/Re-Main-and-interaction-effect-in-Cox-Prop-Hazards-td5738390.html.)

Variables_in_the_Equation.pdf
<http://spssx-discussion.1045642.n5.nabble.com/file/t7186/Variables_in_the_Equation.pdf>  


In a regression model, the terms that you are calling main effects are
really simple effects.  E.g., the coefficient for heavy_drinker shows the
effect of heavy drinking when educ_lowses is set to its reference category.
To get the effect of heavy drinking in the other category of educ_lowses,
add the coefficient for the interaction.  

Similarly, the coefficient for low_ses shows the effect of low_ses when
heavy_drinker is set to its reference category.  To get the low_ses effect
for the other category of heavy_drinker, add the coefficient for the
interaction.  

I worked out those effects in Excel.  Not sure how good the formatting will
be, but I'll paste the results here:

B Heavy Drinking
Low SES Yes No
Yes 0.299 0.393
No 0.566 0.472

If you exponentiate those numbers, you'll get the corresponding Exp(B)
values:

Exp(B) Heavy Drinking
Low SES Yes No
Yes 1.349 1.481
No 1.761 1.603

Finally, the test on the interaction is nowhere near significance (p =
.418).  So unless there is a good theory-based reason for keeping it in the
model, I would consider removing it.  

HTH.



jaycamh wrote

> Hello,
>> I need help.
>>
>> I am doing a Cox proportional hazards model in SPSS. I have Low SES and
>> Heavy drinking as 2 main effects and LOW SES x Heavy drinking as
>> interaction effect in the model.
>>
>> We have stronger (and significant) effects for each main effect but the
>> interaction term is coming protective which is against the theory. Not
>> sure if I am doing it wrong ... see the SPSS result below.
>>
>> I have, Low SES 0 is reference; 1=low SES
>>
>> Heavy drinking 0 is reference; 1=heavy drinking.
>>
>>
>> in fact when I make interaction of alcohol and smoking it’s also coming
>> protective...
>> Is my Interpretation wrong or is there any other way my syntax should
>> have been formed.
>>
>>
>
> Variables in the Equation
>  
> B
> SE
> Wald
> df
> Sig.
> Exp(B)
> 95.0% CI for Exp(B)
>  
> Lower
> Upper
> AGE
> .085
> .000
> 3.719E4
> 1
> .000
> 1.089
> 1.088
> 1.090
> SEX - FEMALE
> -.433
> .013
> 1.179E3
> 1
> .000
> .649
> .633
> .665
> BMI
> .001
> .001
> 6.844
> 1
> .009
> 1.001
> 1.000
> 1.003
> YEAR
> -.006
> .002
> 15.634
> 1
> .000
> .994
> .990
> .997
> smoker_cfn
>  
>  
> 2.716E3
> 2
> .000
>  
>  
>  
> smoker_cfn(1)
> .827
> .016
> 2.600E3
> 1
> .000
> 2.285
> 2.214
> 2.359
> smoker_cfn(2)
> .223
> .015
> 221.735
> 1
> .000
> 1.250
> 1.214
> 1.287
> heavy_drinker
> .566
> .110
> 26.427
> 1
> .000
> 1.761
> 1.419
> 2.186
> educ_lowses
> .393
> .016
> 608.737
> 1
> .000
> 1.481
> 1.436
> 1.528
> heavy_drinker*educ_lowses
> -.094
> .116
> .656
> 1
> .418
> .911
> .726
> 1.
>
>>
>>
>>
>>
>
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Bruce Weaver
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