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Suppose you have SAS code as follows. This is from http://www.ats.ucla.edu/stat/sas/faq/anovmix1.htm.

 PROC MIXED DATA=long;
  CLASS sub group trial;
  MODEL dv = group trial group*trial;
  REPEATED trial / SUBJECT=sub TYPE=CS;
run; 

How would you write the model for this? Would it be:

dv = group + trial +group*trial + subj + error? I am a little confused about how to write the mixed model in terms of a mathematical equation.

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Well, yes. But the key thing in mixed models is that there are two errors; such are more easily represented in matrix form

$Y = X\beta + z\gamma + \epsilon$

see the SAS documentation for MIXED, in particular the "details" section.

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