Re: Error message..

From:"Amir A. Tahami" Subject:Re: [NMusers] Error message.. Date:Thu, March 7, 2002 11:31 am Dear Emily, The variance-covariance matrix (the precision of the parameter estimates) is computed from the R and S matrices [$covariance]. Error messages from the covariance step indicating that these matrices are not positive definite or are singular indicate that the minimization routine may not have found a true minimum. When the S matrix is singular, NONMEM attempts to compute and display a related matrix, the T matrix. This matrix in turn can be used to define a confidence region of the parameter space. Ref.: please see "covariance matrix of estimate" in nmhelp. T matrix helps the geometric transformation, can avoid unwanted translation introduced when we scale or rotate an object not centered at origin. Best regards, Amir A. Tahami