@Cate,   You are welcome!   You can find the information on the SINGULAR= option in the PROC MIXED documentation here. SINGULAR=number tunes the sensitivity in sweeping. If a diagonal pivot element is less than D*number as PROC MIXED sweeps a matrix, the associated column is declared to be linearly dependent upon previous columns, and the associated parameter is set to 0. The value D is the original diagonal element of the matrix. The default is 1E4 times the machine epsilon; this product is approximately 1E–12 on most computers. In your case, the default criterion might be too stringent. That is why I suggested the singular=1e-8 option. ... View more