How to change the reference group in PROC LIFEREG?

[tmleung]

Hi, I am learning SAS. I am running a survival analysis using PROC LIFEREG. The categorical varialbe has 4 groups, namely 1-4. The default reference group is the last group. How can I change it to be the first group?

My code:

proc lifereg data=larynx;
class stage;
model deathtime*death(0)=stage / distribution=exponential;
run;

Thanks

[Reeza]

Try the ORDER  option in the PROC LIFEREG statement. 

[tmleung]

Thanks for your reply.

I tried this

proc lifereg data=larynx;
class stage/ order=data;
model deathtime*death(0)=stage / distribution=exponential;
run;

I also tried just 

class stage/ order;

but still didnt work. and this is what i got

12 class stage/ order=data;
-----
22
202
NOTE: The previous statement has been deleted.
NOTE: PROCEDURE LIFEREG used (Total process time):
real time 0.04 seconds
cpu time 0.04 seconds

NOTE: The SAS System stopped processing this step because of errors.
ERROR 22-322: Syntax error, expecting one of the following: ;, TRUNCATE.
ERROR 202-322: The option or parameter is not recognized and will be ignored.

[Reeza]

Re-read what I posted. 

Order on proc lifereg statement, not class statement. 

[tmleung]

I see....

proc lifereg data=larynx order=data;
class stage;
model deathtime*death(0)=stage / distribution=exponential;
run;

but it still uses stage 4 as my reference group, not stage 1.

[Reeza]

Then unfortunately you'll have to recode the variables in such a way that the last group will be your reference. 

Some Procs have options to change this, but it appears proc LIFEREG doesn't 😫

[tmleung]

OK... Thanks for your help. Really appreciate it.

[Ksharp]

class stage(ref='xxx');

[Reeza]

@Ksharp Unfortunately PROC LIFEREG does not support the REF= option. The only valid option is TRUNCATE, according to the documentation.

http://support.sas.com/documentation/cdl/en/statug/68162/HTML/default/viewer.htm#statug_lifereg_synt...

[Ksharp]

Well. That is bad new. But if it is character, Padding some blank before it to make it happen.

e.x.
want have F be the last level.

if sex='M' then sex='  M';

[FreelanceReinh]

Hello @tmleung,

I've just tested it and found that the ORDER= option of the PROC LIFEREG statement (as suggested by @Reeza) with possible values DATA, FORMATTED (the default), FREQ or INTERNAL works very well.

If ORDER=DATA leaves stage 4 as the reference group, this means that in your input dataset LARYNX stage 4 happens to occur last in the order of observations. So, you have two options:

1. Either you sort dataset LARYNX e.g. by descending STAGE (assuming that this variable has values 1, 2, 3, 4) and then use ORDER=DATA.
   

   proc sort data=larynx;
   by descending stage;
   run;

2. Or you define a format for variable STAGE such that the label of stage 1 comes last in alphabetical order and apply that format to variable STAGE in the PROC LIFEREG step (or assign it permanently in LARYNX), where you also specify ORDER=FORMATTED. Example:
   

   proc format;
   value stagef
   1='Stage 1';
   run;
   
   proc lifereg data=larynx order=formatted;
   format stage stagef.;
   class stage;
   model deathtime*death(0)=stage / distribution=exponential;
   run;

These tips can also be found in http://support.sas.com/kb/37/108.html.

[tmleung]

Thanks.  I have tried to re-order by using PROC SORT but it still takes stage 4 as the reference group.

I will try the PROC FORMAT method later.

Thanks again

[FreelanceReinh]

---

@tmleung wrote:

...  I have tried to re-order by using PROC SORT but it still takes stage 4 as the reference group.

---

This is odd, because in an earlier post you wrote that it also used stage 4 as the reference group with ORDER=DATA, before re-sorting. Sounds like you or your computer made a mistake. 😉

To investigate this, you can use the same ORDER= options with PROC FREQ. The last category in PROC FREQ output should normally be the reference category used by the corresponding PROC LIFEREG step. Example:

proc freq data=larynx order=data;
tables stage;
run;

[tmleung]

You are right. I tried with SAS in another computer and ORDER=DATA works... weird...

Thanks so much for your help.