Hello all,
I'm wondering if
anyone knows of good resource or method for determining sample size needed to
detect a given effect with a given power level in a repeated measures
design (with single factor or multiple factors and levels). For instance,
I would like to be able to make a statement such as: "a sample size of N = ? is
sufficient to detect a 5% difference in variable x with power level of
0.8." The only relevant message I found at the Sport Science website
was from Nov. 5, 2004, but I did not see anything in the message regarding how
to determine sample size in a repeated measures analysis a
priori. I have also consulted the text, Statistical Power
Analysis by Murphy & Myors, 2nd Ed, 2004, but it only mentions
information regarding how and why repeated measures analyses are more
powerful than between-groups designs, with no mention of how to actually
calculate sample size. Finally, Design Sensitivity, by Lipsey,
1990, provides an equation for determining the effect size with paired
samples (which includes the mean difference b/t treatment and control
populations, correlation between paired observations in those populations, and
the common standard deviation of the two populations). This effect size is
then used with a Power chart (intended for independent samples) to determine the
sample size. I am not satisfied with this method, especially for the
small samples (N = 8 or 10) that we typically use for our
research.
Any help would be
greatly appreciated.
------------------------------------------------
Jonathan Wingo
Department of Exercise Science
University of Georgia, USA
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