Re: Model building question

From: "Nick Holford" n.holford@auckland.ac.nz Subject: Re: [NMusers] Model building question Date: Tue, March 1, 2005 4:24 pm Mark, I think you should have read the rest of the paragraph that I wrote before throwing an exception. I was not advocating that all models should be built without thought to sequence. In the particular case at hand I proposed a strategy for building a model based on my prior beliefs of what is important ie. BOV needs to be sorted out first then a fixed effect of occasion. This is the same strategy I would use for adding exploring other covariates i.e. fit the random effect first then the fixed effect e.g. fit the total population parameter variability first (PPV) then add covariate fixed effects in some biologically sensible sequence in order to see if they can reduce PPV. Minimal or no reduction in PPV is a simple performance criterion that can be used to reject inclusion of a covariate despite a moderate fall in OBJ. For clearance I think weight and renal function are primary while race, age and sex are secondary. I use biology to avoid the colinearity trap (e.g. weight and age in children). After building a model with a sensible a priori structure I might then if I had time do some empirical exploratory analysis but I am not a fan of automated blind searches (e.g. including weight in both kg and lb!) :-) Finally, model evaluation should depend on some performance check other than a change in OBJ, covariance step success, etc. Janet's comments on the lack of any performance difference ("The results were similar and would not have resulted in different dosing instructions") despite building different models based on OBJ criteria support this recommendation. Nick -- Nick Holford, Dept Pharmacology & Clinical Pharmacology University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New Zealand email:n.holford@auckland.ac.nz tel:+64(9)373-7599x86730 fax:373-7556 http://www.health.auckland.ac.nz/pharmacology/staff/nholford/