I am a wee bit confused about the random statement in PROC MIXED, I thought maybe you can clarify it for me in simple words.

Let's for the illustration say I have an outcome variable Y, and a treatment variable X, getting:

1 - New intervention

0 - Control (or placebo in drugs)

I have n subjects, and each subject can get the treatment in 1 to 3 places in his/her body.

If I write a SAS PROC MIXED procedure like this:

PROC MIXED data = ....;

CLASS SubjectID, Treatment;

MODEL Y = Treatment;

RANDOM....

run;

what is the difference between:

1) RANDOM SubjectID;

2) RANDOM intercept / subject = SubjectID;

3) RANDOM intercept Treatment / subject = SubjectID;

and when do I use each one ?

thanks !