At 03:19 PM 9/25/01 +0000, Radford Neal wrote: >Neither the question nor the response are all that clearly phrased, but >when I interpret them according to my reading, I don't agree. For instance, >if you're measuring pain levels, I don't see anything wrong with measuring >pain before treatment, randomly assigning patients to treatment and control >groups, doing a regression for pain level afterwards with the pain level >before and a treatment/control indicator as explanatory variables, and >judging the effectiveness of the treatment by looking at the coefficient for >the treatment/control variable. Or is the actual proposal something else?
IMHO seems like to remove the variance from post pain ... using pre pain variance ... is a no brainer ... since the r between the two pain readings will necessarily be high (unless there is something really screwy about the data like severe restriction of range on the post measure) ... what has been explained in the post pain variance? pain? 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) i do most certainly agree with radford that ... random assignment is still essential in this design ... unfortunately, far too many folks use ANCOVA to somehow makeup for the fact that NON random assignment happened and, they think ANCOVA will solve that problem ... it won't > Radford Neal > >---------------------------------------------------------------------------- >Radford M. Neal [EMAIL PROTECTED] >Dept. of Statistics and Dept. of Computer Science [EMAIL PROTECTED] >University of Toronto http://www.cs.utoronto.ca/~radford >---------------------------------------------------------------------------- > > >================================================================= >Instructions for joining and leaving this list and remarks about >the problem of INAPPROPRIATE MESSAGES are available at > http://jse.stat.ncsu.edu/ >================================================================= _________________________________________________________ dennis roberts, educational psychology, penn state university 208 cedar, AC 8148632401, mailto:[EMAIL PROTECTED] http://roberts.ed.psu.edu/users/droberts/drober~1.htm ================================================================= Instructions for joining and leaving this list and remarks about the problem of INAPPROPRIATE MESSAGES are available at http://jse.stat.ncsu.edu/ =================================================================
