Re: Increasing the umber of sub-population in a mixture model

From: "David Nix, Pharm D." <nix@pharmacy.arizona.edu> Subject: Re: Increasing the umber of sub-population in a mixture model Date: Fri, 25 May 2001 13:26:18 -0700 I'm having the same problem with a data set involving administration of the same drug under four conditions to a group of 16 volunteers. The lag time ranges from 0 to 6 hours. Variability of lag time is high within subjects and is not explained by any covariates or by conditions of treatment. The lag time also does not appear to follow any particular statistical distribution. My most recent idea was to provide a fixed and variable component to the lag time. I entered a variable LAG into the data set. LAG was defined as the last time for zero concentration in the inverval between the dose time and the time of first detected concentration minus 15 min. I substracted 15 min since this will allow some room to predict the Tlag without forcing. Values less than 0 were set equal to 0. IF(LAG.EQ.0) ALAG1=0 IF(LAG.GT.0) ALAG1=LAG + THETA(.) * EXP(ETA(.)) I'm currently refining this approach. 1. whether ALAG1 should be fixed at 0 if there is no apparent lag or whether I need allow some variation. 2. whether the distribution for the the THETA portion is additive, proportional or exponential. The exponential error has worked best so far.