RE: Difference between typical values and geometricmea n of posthoc values

From: "Eleveld, DJ" d.j.eleveld@anest.umcg.nl Subject: RE: [NMusers] Difference between typical values and geometricmea n of posthoc values Date: Mon, April 4, 2005 8:47 am Hello everyone, Thank you all again for your helpful reponses to my questions. I seem to have solved my bias problem by using INTERACTION and now the the geometric mean of the posthoc individual parameters is (on average) much closer to the typical values as estimated by nonmem. The root of the problem was my interpretation of when INTERATION should be used. From searching the NONMEM usergroup archives I found the text: If your data appear to have a constant error distribution and this is the same across subjects then you may have a homoscedastic (constrant variance) structure in which case you don't need INTERACTION. I this context I interpreted 'error' as the nonmem ERR() variable which, for my problem, is homoscedastic. So I assumed that INTERACTION shouldnt be used. Unfortunatly, my interpretation was wrong. A better description of when INTERACTION is appropriate might be when heteroscedastic *residuals* are expected. If I had seen it described like this I would have used INTERCTION at the start. Matt Hutmacher and Yaning Wang made it clear in emails to me that you should use INTERACTION whenever the residual term is not additive. This is much clearer to me than what I could find the the NONMEM manuals about INTERACTION. Although I would prefer this to be so plainly stated in the manuals, I guess that it will (with this email) now be archived and searchable in the usersgroup archive so others dont fall into the same trap as I have. Thank you for your help in clearing this up, Doug Eleveld _______________________________________________________