Dble exponential error model

From: laurent.nguyen@pierre-fabre.com Subject: Dble exponential error model Date: Fri, 8 Jan 1999 17:50:46 +0100 Dear NONMEM users, Thanks a lot for these useful informations regarding this odd and unusual dble exponential intra-individual error model for exogeneous substance (that I'm modelling). As Mats or L Sheiner wrote : effectively, I used the following model: Y=F+F.EPS(1) + THETA(10) + THETA(10) .EPS(2) to correct a bias in the lowest observations. I used this model with rich PK data varying over 3 log; and with the lowest values of DV very closed of the limit of assay. As compared with a simple CCV or a slope-intercept intra-individual error model, data seemed better fitted using FO method; especially in the lowest values (less bias in WRES vs Time or PRED plot).In addition, significant but artefactual decrease in objective function (more than 150 points as compared with the slope-intercept error model) resulted using FO approximation. Howewer, as Mats suggested again, I have noted the fact that the constant THETA(10) interferes with other parameters. As a result, residual plots of posthoc estimates (IWRES vs IPRED, etc..) showed largest bias as compared with an intercept-slope error model. Then, not a good model for my data. Best regards, Laurent Nguyen