I would have to respectfully disagree with Dennis' comment also. Having the pre values twice in the model does not hurt or change anything in interpreting the treatment effect.
BUT I do not like this approach. It makes the results more difficult to interpret when you do have a variable in both places. As it is mandatory to have the pre measurement as a separate covariable at any rate, the response variable I prefer is the follow-up assessment, not the change. A good discussion is in Stephen Senn's "Statistical Issues in Drug Development" book (Wiley). -Frank Harrell Radford Neal wrote: > > In article <[EMAIL PROTECTED]>, > Dennis Roberts <[EMAIL PROTECTED]> wrote: > > >the basic idea is to be able to "explain" the post score variance in terms > >of something ELSE ... that is, for example ... we know that some of the > >variance in pain is due to one's TOLERANCE for PAIN ... thus, if we can > >remove the part of pain variance that is due to TOLERANCE FOR pain ... then > >the leftover variance on pain is a purer measure in its own right .. > > > >if you do as suggested ... remove the pre from the post ... say pre pain > >from post pain ... what is left over? it is not pain anymore but rather, > >some OTHER variable ... which is not what the purpose of the study was ... > >to investigate (i assume anyway) > > Well, the idea is that the OTHER variable is the treatment effect, > whose quantification presumably IS the purpose of the study. I think > this is a pretty standard thing to do. > > It seems that the original question was meant to address the more > technical issue of whether you can include the pre-treatment value as > an explanatory variable when the response variable is already the > CHANGE from before treatment to after treatment. As another poster > has ably explained, you can, though it's a bit strange and redundant. > > Radford > > ---------------------------------------------------------------------------- > Radford M. Neal [EMAIL PROTECTED] > Dept. of Statistics and Dept. of Computer Science [EMAIL PROTECTED] > University of Toronto http://www.cs.utoronto.ca/~radford > ---------------------------------------------------------------------------- -- Frank E Harrell Jr Prof. of Biostatistics & Statistics Div. of Biostatistics & Epidem. Dept. of Health Evaluation Sciences U. Virginia School of Medicine http://hesweb1.med.virginia.edu/biostat ================================================================= Instructions for joining and leaving this list and remarks about the problem of INAPPROPRIATE MESSAGES are available at http://jse.stat.ncsu.edu/ =================================================================
