Cox Regression Interaction Interpretation?

02 February 2019 2 3K Report

Hello!

I want to detect whether various diagnosis groups (1,2,3,4) benefit differently from a treatment (1=absent, 2=present).

I have used SPSS and i chose amongst other things "diagnosis group", "treatment" and the interaction "diagnosis group" * "treatment" as predictive markers.

I have selected "last" as reference category and I chose "forward:Wald" as method.

As result I've got among other things these lines:

- Therapy*Diagnosis group Wald:15,368 df:3 significance:0,002

- Therapy*Diagnosis group(1) B: -,151 SE: ,399 Wald:,144 df:1 significance:,704 Exp(B):,860

- Therapy*Diagnosis group(2) B: ,979 SE:,345 Wald:8,064 df:1 significance:,005 Exp(B):2,662

- Therapy*Diagnosis group(3) B:1,084 SE:,371 Wald:8,522 df:1 significance:,004 Exp(B):2,955

I'm not sure how to interpret these results/interactions.

Does it mean that people without therapy who are in diagnosis group 2 have a 166% higher risk to die in the next year than people who received therapy and were in diagnosis group 4?

Does it mean that people without therapy who are in diagnosis group 3 and didn't receive therapy have a risk 195% higher to die in the next year that people in group 4 who did receive therapy?

I would be very grateful for help!

Greetings

Khaled Almaz

https://www.ibm.com/support/knowledgecenter/en/SSLVMB_24.0.0/spss/advanced/idh_coxr.html

https://www.scalelive.com/cox-regression.html

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The steps for interpreting the SPSS output for a Cox regression

1. In the Variables in the Equation table, look at the Sig. column, the Exp(B) column, and the two values under 95.0% CI for Exp(B) column heading.

2. The Sig. column shows the p-value associated with that variable in the model.

3. The Exp(B) column shows the hazard ratio associated with that variable in the model.

4. The values under the 95.0% CI for Exp(B) are the lower and upper limits of the confidence interval for the hazard ratio.

5. Researchers will interpret the hazard ratio in the Exp(B) column and the confidence interval.

If the confidence interval associated with the hazard ratio crosses over 1.0, then there is a non-significant association. The p-value associated with these variables will also be HIGHER than .05.

If the hazard ratio is ABOVE 1.0 and the confidence interval is entirely above 1.0, then exposure to the predictor increases the risk of the outcome.

If the hazard ratio if BELOW 1.0 and the confidence interval is entirely below 1.0, then exposure to the predictor decreases the risk of the outcome.

If the variable is measured at the ordinal or continuous level, then the hazard ratio is interpreted as meaning for every one unit increase in the ordinal or continuous variable, the risk of the outcome increases at the rate specified in the hazard ratio.

David Eugene Booth

SPSS has terrible explanation s of these concepts. Look at the Cox regression material in the attached file. Best wishes, David Booth

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