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1. What is SSP matrix?
2. When I start MCMC estimation a warning message apperas as "SSP matrix for fixedpart has gone negative..." and asks whether to stop estimation or continue. What should I do at this situation?

1) SSP Matrix is short for "sum of squares and products matrix".

2) This and other warning messages are covered in http://www.bristol.ac.uk/cmm/software/s ... rrors.html, although they normally apply to (R)IGLS estimation. It's worth checking that your data has the expected range of values and is sorted in the order of the hierarchy. You could also try a simpler model to see if the issue occurs there.

Thanks a lot