Need dummy variables for Linear Regression

[UGAstudent]

I'm trying to run the proc genmod command, but when I look at level 3, it has 0s across the board but levels 1 and 2 have values. How do I get my level 3 data to show up? I dont know how to code for a dummy variable to get the 3rd level. Any help is appreciated! 

Data UGAStudent_DATA;
Set date_difference; 

if cd4 <= 200 then cd4_levels='1';
if cd4 >=201 <=500 then cd4_levels='2';
if cd4 > 500 then cd4_levels='3'; 
run;

proc genmod data= ugastudent_data;
class cd4_levels /param=glm ;
model  Factor1 =  cd4_levels n_Duration_on_ART_months ;  
run;

[Reeza]

Same answer as before. I'll move this to the Stats forum in case others want to chime in. 

[Reeza · Accepted solution]

Same answers here, for R if that helps at all with the explanation.

This is one of those cases where you need to understand the underlying statistical concepts, specifically: Degrees of Freedom, dummy coding and an over parameterized models. 

Those are some key words you can use to search to explain these results.

https://community.alteryx.com/t5/Advanced-Analytics/In-the-Logistic-Regression-Report-Factor-Missing...

[art297]

As others have sort of mentioned, proc genmod automatically creates dummy variables. And, with any combination of n dummy variables, all of the variance that can be explained is accounted for by n-1 of the dummy variables.

Art, CEO, AnalystFinder.com

[Reeza]

@art297 this is the previous question by the OP

https://communities.sas.com/t5/SAS-Procedures/Data-Missing-on-output/m-p/434932#M68927