asanquer@virbac.fr wrote:
Hi,
I need an advice:
I would like to compare paired observations (pre- and post-treatment
measures)to a fixed value: does the change between pre and post values is
significantly higher than 50?
First I thank to use a classical paired t test, but I would like to use an
adjustment covariate (to take into account an important factor).
What is the correct method to use?
If I use an analysis of covariance:
pre = post + covariate
which part of the results will give me the answer related to the fixed
value 50?
Thanks for your replies
Annaële
The pre-test-post-test design has too many problems to trust the result
(regression to the mean, etc.). Analysis of covariance is a good idea
but you have the pre and post interchanged. And unless you constrain
the slope to be one (which is not a good idea) you can't get a simple
answer to your question but rather need to get several pre level-
specific answers.
If you do want to constrain the slope to one, you can do an analysis not
unlike the paired t-test where the response variable is post-pre. If
you then have a covariate though, the tests will be conditional on
specific covariate values.
You may need a parallel-group design to get a reliable answer to your
question.
--
Frank E Harrell Jr Professor and Chair School of Medicine
Department of Biostatistics Vanderbilt University
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