Re: centering variables

From: "Bruce CHARLES" <bruce@pharmacy.uq.edu.au> Date: Thu, 4 Mar 1999 08:07:21 +1000 Subject: Re: centering variables --In response to Dr. Sheiner's message----- I agree with what you say, except that the statement immediately above in 2. is not quite true, is it? I hope this is not splitting hairs but my interpretation of this always has been that the CL in such "slope-intercept" models has 2 components: Theta(1), a "base" clearance which is independent of age, and Theta(2) a proportionality coefficient for an additional age-dependent CL. Therefore, it's not so much a lack of any "biological meaning" at age 0, its simply that there is an underlying component which contributes to the total CL, independent of age. This is analagous to the contribution of the non-renal component of aminoglycoside CL, and that explained by creatinine clearance, i.e. CL = Theta(1) + Theta(2)* ClCr. The CL reduces to Theta(1) in total renal dysfunction (provided that this pathophysiological state has no influence on non-renal elimination) . Furthermore, when using Model (B) in a mostly elderly population (age distribution skewed to the right) isn't there the risk of getting negative CL values for the younger subjects? Cheers, BC Bruce CHARLES, PhD Senior Lecturer School of Pharmacy The University of Queensland Brisbane, QLD Australia 4072 TEL: +61 7 336 53194 FAX: +61 7 336 51688 Email: Bruce@pharmacy.uq.edu.au