How to create dummy variables for an interval categorical variable to use in proc reg

[cbt2119]

Dear SAS Community,

I am trying to structure my data to run a regression including a categorical variable with multiple interval levels. The variable is viral load control, which was created by looking at lab results across a longitudinal period (VLCONTROL), which has a range of possible values from 1 - 4.

The variable is formatted as a mutually exclusive set of ranges for each response category as defined below.

If all were <50 then vlcontrol=1

If any >=50 and all <400 then vlcontrol=2

If any >=400 and <1000 then vlcontrol=3

If any >=1000 then vlcontrol=4.

I attempted to create a series of dummy variables to run a linear regression in proc reg, but I got the following error.

SAS Output

Note:

Model is not full rank. Least-squares solutions for the parameters are not unique. Some statistics will be misleading. A reported DF of 0 or B means that the estimate is biased.

Note:

The following parameters have been set to 0, since the variables are a linear combination of other variables as shown.

Below is how I coded the dummy variables and how I ran the model:

data regression;
	keep ctvalue vlcontrol vlcont_50 vlcont_400 vlcont_1000 vlcont_ge1000; 
	set have;
	vlcont_50=0;
		if vlcontrol=1 then vlcont_50=1;
	vlcont_400=0;
		if vlcontrol=2 then vlcont_400=1;
	vlcont_1000=0;
		if vlcontrol=3 then vl_1000=1;
	vlcont_ge1000=0;
		if vlcontrol=4 then vlcont_ge1000=1;
	run;

proc reg data=regression outest=est;
	model ctvalue=vlcont_50 vlcont_400 vlcont_1000 vlcont_ge1000/  clb rsquare tol vif aic bic;
	id pid;
	ods output ParameterEstimates=PE;
	run;quit;

Not sure how to do this better. I know I can do this in PROC GLM etc, but I want the diagnostic features for building a model in PROC REG. I've tried not including the last dummy variable, vlcontrol_ge1000, in the model, but still get the same error note in the output. Is there a better way I should be coding my dummy variables?

Thanks,
Cara

[Reeza · Accepted solution]

Generally, when you have 4 levels, you only need 3 dummy variables to accurately represent it, not 4. So you can remove one of your dummy variables, which becomes your reference level and the remaining will be fine.

[cbt2119]

Thanks, this worked! I had to create the dummy variable for the last level and just leave it out of the model for it to work.