Re: Bootstrap resampling!

From: Nick Holford <n.holford@auckland.ac.nz> Subject: Re: Bootstrap resampling! Date: Thu, 29 Mar 2001 13:51:42 +1200 Ludger, Thanks for your insightful comments. You are quite correct that the subject=sampling unit does not randomize the within-subject variability. But we need to distinguish exactly what this involves. There is true within-subject variabilty which we might model as between occasion variability if the subject is studied on more than one occasion. This is an important level of variability that would need to be considered but I do not know how to arrange the sampling to do it. There is also residual unknown variability (also unfortunately called intra-individual variability). This is of course nearly always modelled quite independently of the individual so it seems wrong to call it intra-individual when it is estimated from residual error across all individuals. Once again it is hard to know how to sample this and still preserve the heteroscedasticity that is so typical of PK data. What assumptions do you want to make for a reasonable method of sampling? Nick -- Nick Holford, Divn Pharmacology & Clinical Pharmacology University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New Zealand email:n.holford@auckland.ac.nz tel:+64(9)373-7599x6730 fax:373-7556 http://www.phm.auckland.ac.nz/Staff/NHolford/nholford.htm