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Hi All, I am implementing the zero trick for a non-standard prior for a variance matrix. The matrix is restricted because of some symmetry conditions which results in the restriction on the values of correlation parameters. I have used the zero-trick (on the sample space for correlations) to implement the condition that the variance matrix be positive-definite. The zero trick is supposed to help in the generation of values of the correlation parameters so that only legal values are generated. What happens is that sometimes illegal values are picked up and as a result the updater stops with a trap message because the variance matrix generated is not positive definite. Interestingly, the code runs fine in the beta version of OpenBUGS (2007) but not in WinBUGS1.4 and the current version of OpenBUGS. Anyone has an insight about how the zero-trick is implemented? My impression is that it is part of the prior (like censoring/truncation). Paramjit Gill ------------------------------------------------------------------- This list is for discussion of modelling issues and the BUGS software. For help with crashes and error messages, first mail [log in to unmask] To mail the BUGS list, mail to [log in to unmask] Before mailing, please check the archive at www.jiscmail.ac.uk/lists/bugs.html Please do not mail attachments to the list. To leave the BUGS list, send LEAVE BUGS to [log in to unmask] If this fails, mail [log in to unmask], NOT the whole list