topic Re: reference values in GENMOD v8.2 in Statistical Procedures

reference values in GENMOD v8.2

MarthaAnne — Tue, 31 May 2011 20:53:54 GMT

I'm working in an OpenVMS batch environment which is still running SAS v8.2. In this version, PROC GENMOD does have a CLASS statement, but this statement doesn't have any options. So I can't specify reference values for the covariates with, for example, (ref='a'). I've been unable to find anything in the v8 documentation explaining what SAS uses for reference values for binary, continuous, or categorical covariates. Does anyone know?

Before you suggest it, upgrading is not an option. Also, I'm not a statistician, I just play one at work. So go easy on the jargon.

Thanks!

Re: reference values in GENMOD v8.2

MattF — Wed, 01 Jun 2011 12:17:55 GMT

Hi:

A time-honored approach is to use formats - with the default descending order, a prefix of "z." on your desired category will force it to be last in the sort order, hence the base class (assuming 0/1 design matrix coding).

/*default "M" is base class */
proc genmod data=sashelp.class;
class sex;
model height = age sex;
run;

/* forcing "F" to be the base class */
proc format;
value $sex
"F" = "z.F"
;
run;

proc genmod data=sashelp.class;
class sex;
model height = age sex;
format sex $sex.;
run;

Best regards,
-Matt

Re: reference values in GENMOD v8.2

MarthaAnne — Wed, 01 Jun 2011 17:41:20 GMT

Matt,
Thanks for the quick response. My question was even more basic, but I think you answered by implication. Can I assume that SAS sorts formatted values alphabetically and then uses the last one as the reference? Is this true for all variable types (binary, numeric, character)?
-- mc