Re: Residuals

From: KENNETH.G.KOWALSKI@monsanto.com Subject: Re: Residuals Date: 08 Oct 1998 10:53:23 -0500 Lewis writes: >Residuals are very useful for model-building, to check whether the >model descibes the data adequately. But checking the distributional >shape is not a particularly helpful procedure for this purpose... Although as Lewis indicates the assumption of normality is not required and more often than not the only thing we need to worry about is bias (mean of etas near zero), I think there is some value in looking at the distributional shape of the etas. Histograms of the etas is a standard plot I look at in my battery of diagnostic plots. I have one example where bimodality was observed in a histogram for a particular eta. I could not identify a covariate to explain the bimodal distribution hence, I used a mixture model (see Guide VII, pp.8-9) to assign subjects to one of two subpopulations using the MIX routine in NONMEM. This worked out really nicely. Ken