Re: Probabilistic model

From: jeffrey.a.wald@gsk.com Subject: Re: [NMusers] Probabilistic model Date: Wed, May 18, 2005 8:33 am You cannot throw away information you do not possess. If you have a 6 category scale but a few of the categories are not populated with a sufficient number of observations, then combing them is perfectly valid and will add stability to the final solution. A bigger danger in my mind is to assume that you can extrapolate, on the basis of arbitrarily converting categories to continuous responses, to nonobserved responses. This is not necessarily a function of the number of categories. Take an 11-point pain scale. You might have very robust (and apparently continuous data) in the high to middle range of the scale. Now treat patients with an mildly effective drug. Absent a large placebo response, you are just not going to see enough of the 0's, 1's and 2's to resolve individual probabilities for these scores. As a friend and erstwhile mentor would say, "there is no substitute for no data". (I am still trying to figure that one out :-) Jeff Jeff Wald, PhD jeffrey.a.wald@gsk.com Clinical Pharmacokinetics/Modeling and Simulation Neurology and GI RTP, NC