Re: [R] use "factor" for categorical covariate in Cox PH model

From: Dieter Menne <dieter.menne_at_menne-biomed.de>
Date: Tue 18 Jul 2006 - 17:09:36 EST

Linda Lei <llei <at> bccrc.ca> writes:

>
> When dealing with the categorical covariates (for example 3 groups), it
> will come out different results if we add the command "factor" in front
> of the categorical covariate or not

The catch is here: the covariate is not a "categorial", but a number. In that case a regression is computed, and the ONE ration printed out has the meaning of a slope.

> if we don't add "factor", there is
> only one hazard ratio; if we add "factor", there are two hazard ratios.
>
> So does the "factor" actually create some dummy variables for the
> calculation?

Factor make the covariate a categorial one, and two contrasts are printed out, relative to the first level of the covariate.

Dieter



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