I have just been reviewing a study proposal.
Two (medical) treatments are to be compared, and the primary efficacy
measure is a continuous variable expected to have a non-normal distribution.
The primary analysis is to use a Wilcoxon test to compare the treatments.
Sample size was calculated, correctly, to have 80% power when p1 = 0.75,
where p1 is the probability that an observation in group 1 will be less than
an observation in group 2. (This is the nomenclature and definition used by
nQuery)
I have two questions for the list
1. Is there a short name for p1 defined as above, or some variant on it?
2. Do others share my concern that p1 is not the way most people will think
about differences between treatments (although it is the obvious parameter
to use to power a Wilcoxon test), which mean it is almost impossible to
assess whether p1 = 0.75 is an appropriate effect size to design for.
Responses to me please, and I will summarize to the list.
Thanks
Tim Auton
The views, opinions and judgements expressed in this message are solely
those of the author. The message contents have not been reviewed or
approved by Protherics.
T R Auton PhD MSc C.Math
Head of Biomedical Statistics
Protherics Molecular Design Ltd
The Heath Business and Technical Park
Runcorn
Cheshire
WA7 4QF
UK
email: [log in to unmask]
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