I might have completely missed the point, but why is it that you have standardized the testosterone levels? If total testosterone is free testosterone + some other testosterone, then one can imagine that the SD of total testosterone is greater than that of free testosterone. Given this, the HR per raw score might even be greater for free testosterone than total, since HR per raw score = HR per z score / SD. But there may be very good reasons for standardizing...
Tim
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Zoe Hyde
Sent: 21 January 2009 04:34
To: [email protected]
Subject: st: How can I compare two Cox models - can I use Harrell's C/ROC curves?
Dear All,
I have two Cox models that I am using to predict stroke. One model includes total testosterone (HR=2.0) and the other free testosterone (HR=1.7), as Z-scores, along with some other covariates. The hazard ratios seem to suggest that changes in total testosterone are more important than free testosterone, but is this really the case? Is there some way that I can compare the two models, for example by using ROC curves?
I have used estat con to obtain Harrell's C for both models, and there doesn't seem to be a lot of difference - 0.69 for one model and 0.67 for the other. However, I'd feel more comfortable if there was some way I could test the significance of the difference. Is there is test I can use to compare these two results?
Any advice would be much appreciated!
Thanks,
Zoe.
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