When you create dummy variables, you need to create one less than the number of levels. The reason for this is if you know 2, you can always figure out what the third value should be, so it doesn't add any information. Technically, in linear algebra terms, it means that the third indicator is a linear combination of the first two indicator variables so you need to drop one variable.
I would recommend switching to PROC GLM and using a CLASS statement instead though.
proc reg data=labla (rename = class=grade_level);
where grade_level in (6, 7, 8);
class grade_level(ref=first) / param=ref;
Model score = grade_level;
Run;
Assuming the code you've shown is correct and works without errors in the log the above code should work and provide what you want, it will be comparing grade 7/8 to grade 6 though.
@tuntun wrote:
I want to compare different data in 1 variable (class).
For example, i want to compare student in class 6, class 7 and class 8 with another variable, which are student's scores, so i code like this in new data set:
If class = 6 then class_6 =1 (class 6 is new variable)
Else If class = 7 then class_6=0
and i do similar for class 7 and 8;
my proc reg :data=labla;
Model score = Class_6 class_7 class_8;
Run;
However, I get this not when use proc reg , i think because there are many class beside 6,7 and 8 in variable class and I just want to compare these three, which lead to this 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.
what can i do to solve this problem, is there any better way to find out and compare score of class6,7,8 besides proc reg, thank you.
