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st: xsampsi produces inconsistent results


From   Michael McCulloch <mm@pinestreetfoundation.org>
To   Statalist Statalist <statalist@hsphsun2.harvard.edu>
Subject   st: xsampsi produces inconsistent results
Date   Sat, 5 Jun 2010 18:37:08 -0700

Hello,
I'm using -xsampsi- to estimate sample sizes for a crossover experiment.
I compared two examples, each with alpha=0.25, and beta=0.20.

Experiment 1 (where mean difference =30, and its standard deviation = 45):
	xsampsi, alpha(0.025) beta(0.20) delta(30) stddev(45)

Experiment 2 (where mean difference =2.3, and its standard deviation = 0.63):
	xsampsi, alpha(0.025) beta(0.20) delta(2.3) stddev(.63)

The first example produces a result, but the second leads to error message:
	"Need to install package 'nct' to compute non-central t distribution"

However, I have installed -nct-.

Any feedback would be appreciated!



Best wishes,

Michael McCulloch, LAc MPH PhD
Pine Street Foundation
124 Pine Street
San Anselmo, CA 94960-2674
tel:	415-407-1357
fax: 	206-338-2391

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