At 10:26 AM 9/25/01 +0000, Morelli Paolo wrote: >HI all, >I have to analyse some clinical data. In particular the analysis is a >comparison between two groups of the mean change baseline to endpoint of a >score. The statistician who planned the analysis used the ANCOVA on the mean >change, using as covariate the baseline values of the scores. >Do you think this analysis is correct?
NO! ... this is not a legitimate covariate ... a pre measure of the same thing you are measuring latter as evidence of effectiveness the notion of a covariate is to have previously collected data ... on a variable that rationally should explain some of the variance in the criterion ... and the idea is to "remove" that part of the criterion variance that can be accounted for by the co-linearity with the covariate in situations where the treatment effect is likely to be small ... especially if error variance is large ... using an appropriate covariate (assuming of course that Ss were randomly assigned to the different conditions) is a good way to reduce the error term and hence, increase your chances for finding "significance" (if that is your goal) >I thing that in this way we are correcting twice. I think that the right >analysis is an ANOVA on the mean change. >Please let me know your opinion >thanks >Paolo > > > > >================================================================= >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/ =================================================================
