topic Re: How to change the reference group in PROC LIFEREG? in Statistical Procedures

How to change the reference group in PROC LIFEREG?

tmleung — Wed, 08 Jun 2016 20:40:59 GMT

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

Re: How to change the reference group in PROC LIFEREG?

Reeza — Wed, 08 Jun 2016 21:11:20 GMT

Try the ORDER option in the PROC LIFEREG statement.

Re: How to change the reference group in PROC LIFEREG?

tmleung — Wed, 08 Jun 2016 21:24:28 GMT

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.

Re: How to change the reference group in PROC LIFEREG?

Reeza — Wed, 08 Jun 2016 21:27:47 GMT

Re-read what I posted.

Order on proc lifereg statement, not class statement.

Re: How to change the reference group in PROC LIFEREG?

tmleung — Wed, 08 Jun 2016 21:36:03 GMT

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.

Re: How to change the reference group in PROC LIFEREG?

Reeza — Wed, 08 Jun 2016 21:38:55 GMT

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 😫

Re: How to change the reference group in PROC LIFEREG?

tmleung — Wed, 08 Jun 2016 21:40:03 GMT

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

Re: How to change the reference group in PROC LIFEREG?

Ksharp — Thu, 09 Jun 2016 03:21:52 GMT

class stage(ref='xxx');

Re: How to change the reference group in PROC LIFEREG?

Reeza — Thu, 09 Jun 2016 03:48:05 GMT

@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_syntax04.htm

Re: How to change the reference group in PROC LIFEREG?

Ksharp — Thu, 09 Jun 2016 04:07:24 GMT

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';

Re: How to change the reference group in PROC LIFEREG?

FreelanceReinh — Thu, 09 Jun 2016 08:42:47 GMT

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:

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;
```
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.

Re: How to change the reference group in PROC LIFEREG?

tmleung — Thu, 09 Jun 2016 13:44:43 GMT

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

Re: How to change the reference group in PROC LIFEREG?

FreelanceReinh — Thu, 09 Jun 2016 15:51:44 GMT

@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;

Re: How to change the reference group in PROC LIFEREG?

tmleung — Thu, 09 Jun 2016 16:00:31 GMT

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

Thanks so much for your help.