I am curious as to why NLME sets a diagonal omega matrix instead of a full block omega matrix as the default when first attempting to converge a population PK model. Wouldn't it be best to first attempt converging a model that has a full block omega matrix (aka off diagonal covariances estimated as well as variances) and then check the estimated covariance values to confirm that there is negligible (say less than 0.0025) instead of setting a diagonal omega matrix from the start?

Dan