The Z matrix is constructed exactly as the X matrix for the fixed effects, and the G matrix is constructed to correspond with the effects constituting Z . All unspecified locations are assumed to be 0. The second representation is for dense matrices. See Example 41.4 .
All observations having the same level of the group effect have the same covariance parameters.
Each new level of the group effect produces a new set of covariance parameters with the same structure as the original group. You should exercise caution in defining the group effect, as strange covariance patterns can result with its misuse. Also, the group effect can greatly increase the number of estimated covariance parameters, which may adversely affect the optimization process. Unspecified values of these matrices are set equal to 0. By default, these columns are included in Z .