Hi, Thanks so much for your comments, I think Steve's point about the colinearity between my variables is true. I think the reason for singularity problem and getting infinite values for z scores is the colinearity between v10, v13, and y. Of course, I have used the interactions of v13 with some other variables, and probably some interactions are colinear with v10 and y. Now, I see why scaling some variables creates the singularity and infinite z scores problems, because it makes their interactions with v13 be more correlated with y when they are scaled. Am I right? Also, I have used 8 continuous variables as random effects, and the rest of them are binary variables (not dummy though). Do you have any suggestions to solve this problem and get reasonable responses? Thank you, Mohsen
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