RE: order of covariate inclusion

From: sduffull@pharmacy.uq.edu.au Subject: RE: [NMusers] order of covariate inclusion Date: 9/25/2003 6:14 PM Hi Implementation of covariate model building in this manner: >> Forward addiction procedure is implemented as follows: >> If you have covariates A, B, and C, you test all of the >> models Base + A Base + B Base + C and then choose the best >> model (say, Base +A, if Base +A is significantly >> better than Base and better than Base+B) ... does provide initial models that are not nested. The 'sequential' stepwise approach generally avoids this problem (i.e. adding a additional covariates sequentially [in theory] will produce a sequence of nested models). You can of course also consider biological plausibility as an additional guide to choice of initial covariates - for example the effects of adding a covariate describing male/female can often be described completely by consideration of patient weight. Regards Steve ========================================= Stephen Duffull School of Pharmacy University of Queensland Brisbane 4072 Australia Tel +61 7 3365 8808 Fax +61 7 3365 1688 University Provider Number: 00025B Email: sduffull@pharmacy.uq.edu.au www: http://www.uq.edu.au/pharmacy/sduffull/duffull.htm PFIM: http://www.uq.edu.au/pharmacy/sduffull/pfim.htm MCMC PK example: http://www.uq.edu.au/pharmacy/sduffull/MCMC_eg.htm