Re: More Levels of Random Effects

Hi Nick; In your example, I would also do the latter, since I would have a rough idea of the proportion of each of those groups living in the UK. But, I am not sure your example is analagous to IOV, principally because, in your example of Scots, Irish and Welsh, time is not involved. If you have 4 occasions, and you fit 4 variability terms as separate (no BLOCK SAME) don't they now become ordered categories? If there is a definite pattern to the variability changes as a function of time, wouldn't that change be better modeled explicitly as a time-dependent change, rather than in an implicit way? I think your suggestion of running simulations based on the 1-4th occasion when extrapolating is reasonable; however, the number of simulations that might have to be performed may, in certain cases quickly add up. Mike Fossler GSK "Nick Holford" <[EMAIL PROTECTED]> Sent by: [EMAIL PROTECTED] 17-Oct-2008 15:38 To "nmusers" <[email protected]> cc Subject Re: [NMusers] More Levels of Random Effects Mike, So how do you deal with other non-ordered categorical variables? Suppose you do your studies in Scotland, Ireland and Wales then need to predict what will happen in England? Assuming you found 'significant' differences in between subject variability in clearance between the Scots, Irish and Welsh and wanted to predict a population in England do you think it would better to take the average of the Scots, Irish and Welsh (equivalent to using SAME) or do you think it would be better to randomly choose from the 3 groups knowing that representatives of these 3 groups might be found living in England? I would think the latter approach would be more realistic. I would consider doing something similar for between occasion variability (aka IOV) if I find 'significant' differences across 3 occasions and need to predict a study which has 4 occasions. Rather than assume the 4th occasion is the average of the other 3 I would consider randomly assigning the 4th occasion data item to 1, 2 or 3. Nick [EMAIL PROTECTED] wrote: > > > I suppose it really comes down to what you are going to do with the > model. Many times I have checked the SAME assumption when modeling > inter-occasional variability, and found that sometimes, removing it > does indeed improve the fit significantly. In almost every case I've > retained it (despite the better fit) for the exact reasons Leonid > cites: it makes your model completely data-dependent. I suppose if the > model was meant as a description or summary of the data, then it would > not matter, but I make all of my models work for a living... > -- Nick Holford, Dept Pharmacology & Clinical Pharmacology University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New Zealand [EMAIL PROTECTED] tel:+64(9)923-6730 fax:+64(9)373-7090 http://www.fmhs.auckland.ac.nz/sms/pharmacology/holford