Re: Probabilistic model

From: "Nick Holford" Subject: Re: [NMusers] Probabilistic model Date: Tue, May 17, 2005 7:41 pm Chuanpu et al, Thanks for taking the bait :-) I knew you and your statistical colleagues would take me to task for even thinking of treating categorical variables as continuous. While I have heard this caution many times before I have never seen a numerical example which illustrates the practical consequences given some realistic example. There are other examples of statistical 'knowledge' that have not been borne out when examined by experiment (e.g. distribution of delta OBJ under the null, meaningfulness of getting covariance step to run when assessing parameter reliability) so I wonder if anyone has done any work with NONMEM in this area? Many uses of categorical variables in drug development reflect naive attempts by investigators to capture what is really a continuous scale variable e.g. pain, neutropenia. IMHO such categorical scales are intepreted by those who look at the results as if they were indeed a continuous scale variable. It seems quite reasonable if you have even a 5 point categorical scale to consider this as continuous. Depending on the a priori knowledge of the system you might choose to fix the residual error on each category to some reasonable value e.g. a pain score on a 5 point scale might have a residual SD of 0.5 units (i.e. 3 is usually clearly different from 2 or 4). Nick