PROC PHREG - with CLASS statement

MTeck — Sun, 21 Jul 2019 06:02:14 GMT

i am wondering either i add "CLASS" statement or not.

i am trying to run Cox-regression model, so i made this code.

and what i need is the hard ratios for outcome on exposure.

exposure(0=no exposure, 1= yes exposure) and outcome(0=no outcome, 1= yes outcome) variable are all binary.

Can i add class statement to want to see hazard ratios on exposure

proc phreg data=episode;

/*class exposure*/
model period*outcome(0)=exposure / rl;
run;

My output is per individual,

1) class statement

Parameter

Estimate

-1.45709

Standard

Error

0.15452

Chi-Square

88.9231

> ChiSq

<.0001

Hazard

Ratio

0.233

95% Hazard Ratio

0.172

ConfidenceLimit

0.315

2) non class statement

Parameter

Estimate

Standard

Error

Chi-Square

> ChiSq

Hazard

Ratio

95% Hazard Ratio

ConfidenceLimit

1.45709

0.15452

88.9231

<.0001

4.293

3.172

5.812

Re: PROC PHREG - with CLASS statement

FreelanceReinh — Sun, 21 Jul 2019 09:22:38 GMT

Hello @MTeck and welcome to the SAS Support Communities!

In the case of a dichotomous explanatory variable with values 0 and 1 (like exposure in your data) the results with vs. without a CLASS statement are essentially the same. However, it can happen (and it did in your example) that the CLASS statement uses level '1' of that explanatory variable as the reference level so that the sign of the corresponding parameter estimate changes and the inverse hazard ratio and confidence limits are computed, here: the hazard ratio of "no exposure" vs. "exposure."

I would use the CLASS statement (because exposure is a classification variable) and explicitly specify the reference level so that the intended results are clear. If variable exposure is not formatted:

class exposure(ref='0');

If variable exposure is formatted and the formatted value of exposure=0 is 'no':

class exposure(ref='no');

Or, to avoid hardcoding of formatted values:

class exposure(ref=first) / order=internal;

(Among the internal values of exposure, 0 and 1, 0 is the first, regardless of formats. See the documentation for more details.)

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PROC PHREG - with CLASS statement

Re: PROC PHREG - with CLASS statement