Re: ADVAN 6

From: alison@c255.ucsf.EDU (ABoeckmann) Subject: Re: ADVAN 6 Date: Wed, 7 Oct 1998 15:24:17 -0700 (PDT) Here are some general guidelines. The number 100000 was chosen arbitrarily to prevent endless loops when the step size for an integration is driven to very low values, effectively 0. It can stop a run when the convergence is slow. It is possible that convergence is slow because the model is not very good. If you cannot improve the model, and it appears that the search was indeed making reasonable progress towards a mininum, here are three suggestions. You can try ADVAN8 and/or ADVAN9 instead of ADVAN6; either of these is better able to handle stiff systems of differential equations. You can divide the long run into several smaller runs, using the MAXEVALS option of the $ESTIMATION record. Use Model Specification files so that each run is continued by the subsequent run. This will keep each run under the limitation of 100000 evaluations of the differential equations. Alternately, you can increase this limit. In your $PK block, include the following verbatim code: $PK "FIRST " COMMON /PRCOMG/ IDUM1,IDUM2,IMAX,IDUM4,IDUM5 " INTEGER IDUM1,IDUM2,IMAX,IDUM4,IDUM5 " IMAX=200000 The constant 200000 gives you twice the default; you may use a larger or smaller value. STeve also says: > Sometimes it claims to have successfully converged - although when looking > at the fits this does not appear to be the case. Any ideas how to get > around this one? I think this is another question, right? The error message never appeared, the run appears to be ok, but the fit is biased. It is a question that is not easy to answer, but here are a few brief ideas. Biased fits are discussed somewhat in Guide VII. Its possible that METHOD=1 (FOCE) may help. Run times and numeric difficulties will be much worse. If the variance of most of the etas is small and only one or two have large variances (and considerable variability in the posthoc etas), consider the HYBRID method, in which the well behaved etas are held to 0 (using the ZERO option) and only the badly behaved etas are estimated by the FOCE method. For information about HYBRID and ZERO, see Guide VIII ($ESTIM). See also Guide VII. -- Alison Boeckmann