Re: Model building question

From: "kai wu" kaiwu77@yahoo.com Subject: Re: [NMusers] Model building question Date: Mon, February 28, 2005 3:59 pm Dr. Holford, I surely understand the importance of the BOV. However, my doubt is on how should I incorporate it into my model building process. As you suggested, "You could try estimating both the mean change in each parameter relative to the first occasion and on top of that estimate the BOV as the true random differences between the occasions", if I understand right, I should decide the proper covariate model for Thetas first, then add BOV to account for the random difference between occasions. That is exactly what I did. The reason that I had doubt is that I remember that I came across a paper, where the BOV was incoporated into the base model first, and as paper stated, since BOV was determined not significant, it was disregarded for further covariate model building for thetas. I also tried this approach, and my data showed none of BOVs was significant. However, the first approach (adding BOV at last) would support BOV for Vd was significant, and the coefficients for the covariate model of theta would be different in two approaches too. The second question is that what is the criteria to judge BOV significant or not? I was only comparing OBJ values and diagnostic plots. As I understand, NONEM was putting BOV into residual error when modelling w/o BOV. In my case, I only found the residual error decrease from 31% to 29% after adding BOV. Kai