What is it about?
Correlation between the test for carry-over and the test for treatment based on first period values only strongly biases the significance test for treatment if based on the test for carry-over, as was shown by Peter Freeman in 1989. This is explained and illustrated in this viewpoint piece.
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Why is it important?
It shows that preliminary testing to guide choice of analysis can be dangerous and that the properties as a whole of such procedures should be analysed.
Perspectives
I was a reviewer of Peter Freeman's article for Statistics in Medicine and had independently come to the conclusion that there was a problem with the procedure. Peter had analysed the issue in depth and it was a very simple matter to recommend publication.
Professor Stephen J Senn
Consultant Statistician
Read the Original
This page is a summary of: Viewpoint
: Do not resurrect the two‐stage procedure, Pharmaceutical Statistics, July 2022, Wiley,
DOI: 10.1002/pst.2224.
You can read the full text:
Resources
Testing Times
A blog explaining the problem with the tow-stage procedure
The AB/BA Cross-over: How to perform the twostage analysis if you can’t be persuaded that you shouldn’t.
This paper, which is otherwise difficult to get hold of, describes how the two-stage procedure can be altered to maintain the nominal error rate.
Contributors
The following have contributed to this page
