Re: Inclusion of covariates and model improvement

From: Stephen Duffull steveduffull@yahoo.com.au Subject: Re: [NMusers] Inclusion of covariates and model improvement Date: Thu, 20 Oct 2005 17:38:14 +1000 (EST) Toufigh. I think there are 2 issues here that can be considered: 1) What did you write in your analysis plan? If you wrote that the OBJF and BSV (IIV in your notation) should decrease when a covariate is included and this has happened then why not include the covariate? Certainly as your model gets more complex then SE's may well increase since there is a finite amount of information in a design which has to be distributed amongst the parameters of interest. It would be interesting to see whether centring your parameter by standardising your weight to 70kg (or whatever number) may result in lower standard errors. 2) Does inclusion of the covariate help explain some drug effect of interest; i.e. would you alter the dosing regimen based on inclusion of a PK covariate? When considering this latter case it is important to remember that patient demographics in clinical studies are invariably not as extreme as is seen in clinical practice. Regards Steve