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?
> 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
It's convoluted, but not wrong. I do it sometimes because some
researchers, for whatever reason, are more comfortable with that
approach. The research question is usually: If two people have the same
initial value, will there final value be the same except for the effect
of treatment. (I'm assuming your groups are the result of random
assignment to treatment. If not, these arguments does not apply and I
leave it to you to read the literature to find out why. I'm quickly
using up my daily allotment of keystrokes!) This gets you the ANCOVA
model
final = constant + b1 * initial + treatment effect
Change is final - initial, so the model can be rewritten
change = constant + (b1-1)* initial + treatment effect
and the estimated treatment effect is the same. Since the treatment
effect is the same, the analysis is okay, odd as it looks.
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