SAS day 17: Proc Phreg

One day, my boss took a glance at a table with Hazard Ratio and Median Survival Time then he told me the program set the reference group in Proc Phreg wrong.

It turns out he was correct after validating the program. However, I was very curious about how did he figure it out by an Augenblick. Then he shared with us some knowledge about Hazard Ratio and Proc Phreg.

Background knowledge

Hazard Ratio: HRs represent instantaneous risk over the study time period.

Median Survival Time: The time after which 50 percent of people with a particular condition are still living, and 50 percent have died.

Median survival time and Hazard Ratio

if Median(A) > Median(B) then HR(A) < HR(B), vice versa.

Without Loss of Generality, if the median survival time of group A is significantly higher than group B, then Hazard Ratio of group A is smaller than Hazard Ratio of group B, vice versa.

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qimono / Pixabay[/caption]

**Example  **

Now we will demonstrate Proc Phreg with Hazard Ratio with Sashelp.BMT dataset.

Basic Proc Phreg Syntax

Proc phreg data=a;
class x ; /*ref(x) only character value, the lowest character value is reference*/
model x*y= a/rl ; /*can input more than one variable on the RHS of the equation. rl:risk limit (eßt)*/
by z;/*optional*/ 
id m;/*optional*/
run;

1. Read in the sashelp.BMT dataset and only consider AML Low Group and AML High Group
   Note: Hazard Ratio is usually designed for two-level comparison,
   we will introduce 3-level comparison next time.

data aml;
set sashelp.bmt;
if group="AML-Low Risk" then ref="0";
if group="AML-High Risk" then ref="1";
if ref in("0","1");
run;

2.  Apply Proc Phreg to generate the hazard ratio

proc phreg data=aml outest=est61 covout;
class ref;
model T*status(0)=ref/rl;
run;

Results:

From the output, we have Hazard Ratio=0.39, which means the probability a patient in AML high Group survives until today and will survive until tomorrow, compared with the AML Low Group Reduced by 0.39.

Validation:

As we mentioned earlier, if Hazard Ratio <1 then we expected a longer survival median time in AML Low Group. The following Kaplan Meier plot demonstrated the AML Low Group indeed has a larger survival median.

proc lifetest data=aml plots=survival;
time T*status(0);
strata Group;
run;

Thanks very much to Joe wang to share the Proc Phreg knowledge with me, we will show the 3-level Hazard Ratio in Proc Phreg next time!

Happy studying! 🐰