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Re: st: Re: sample size calculation


From   Richard Williams <Richard.A.Williams.5@nd.edu>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Re: sample size calculation
Date   Wed, 29 Sep 2004 22:52:25 -0500

At 12:28 PM 9/30/2004 +0900, Joseph Coveney wrote:
It seems as Ricardo's authors are assaying nicotine or its metabolites in some
tissue (placenta, perhaps). In pharmacokinetics, variation often *is*
expressed in terms of the coefficient of variation, as Clint suggests. If
Ricardo's authors intended to say that the standard deviation is 15% instead of
15 ng/mg, and if they contemplated using Bonferroni's inequality to adjust
alpha for two comparisons among the three groups, then

-sampsi 100 115, alpha(0.025) power(0.8) sd1(15) sd2(15)-

gives 43. Here, I'm assuming that power, not beta, is 80%. It's possible that
Ricardo's authors' article is peppered with unintentional errors and omissions
in describing things, such as power and the scale for variation.
I think you meant

sampsi 100 110, alpha(0.025) power(0.8) sd1(15) sd2(15)

At least, that is what gives me an N of 43; the original command gives Ns of 20.

Yes, part of what confuses me about the original problem is the use of both percents and non-percents. You get the feeling something is missing or misstated; but then I am far more used to doing secondary analysis of large data sets than I am problems like this. I suppose another possibility is that the 15 ng/mg is correct but then it gets converted into some sort of percentage?


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Richard Williams, Notre Dame Dept of Sociology
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