Re: Probabilistic model

From: "Nick Holford" n.holford@auckland.ac.nz Subject: Re: [NMusers] Probabilistic model Date: Thu, May 19, 2005 10:47 pm Jeff, jeffrey.a.wald@gsk.com wrote: > > You cannot throw away information you do not possess. But if you have information and merge it with other information without keeping track of the original state then information must be lost. You CAN throw away information that you do possess --- but it isn't a good idea. This is why I do not like the idea of combining categories. > 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. > This is a different issue. The design may indeed make it hard to identify certain levels of response but this is a problem for continuous as well as categorical analysis too. Nick -- Nick Holford, Dept Pharmacology & Clinical Pharmacology University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New Zealand email:n.holford@auckland.ac.nz tel:+64(9)373-7599x86730 fax:373-7556 http://www.health.auckland.ac.nz/pharmacology/staff/nholford/ _______________________________________________________