Re: Dble exponential error model

From: Mats Karlsson <mats.karlsson@biof.uu.se> Subject: Re: Dble exponential error model Date: Thu, 07 Jan 1999 23:50:01 +0100 Dear Laurent and others, Although the answer I (and Steve and Lewis) gave on the problem below offered correct slope-intercept models, they didn't realize that the model suggested actually was a valid (although unusual) one. It is not unidentifiable. One obvious example is if there are measured concentrations where the PK model (via F) predicts none, e.g. with an endogeneous substance. However, from what he writes I don't think Laurent meant such a situation, but rather one where the additive component (THETA(10)) should take care of a positive bias in the assay and improve the error model for low observations. To get IWRES for such a model, the following code could be used: W=SQRT(THETA(11)**2 + F**2*THETA(12)**2) ;THETA(11)=SD(additive) THETA(12)=CV(proportional) IWRES=(DV-RES)/W Y=F+THETA(10)+W*EPS(1) $SIGMA 1 FIX However, I agree with the misgivings of Steve and Lewis earlier that for an exogeneous substance it is an odd error model. The risk is that the constant (THETA(10)) will intefere with the estimation of e.g. the terminal slope. Best regards, Mats