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Re: st: Computing effect size for a clustered randomized control trial

From   Jeph Herrin <>
Subject   Re: st: Computing effect size for a clustered randomized control trial
Date   Mon, 03 May 2010 21:46:22 -0400

you can try -findit sampclus-

but whatever method you use, you will need an estimate of the
intraclass correlation.


Joe McCrary wrote:
I am designing a randomized control trial, where we are going to
select about 12 students per classroom and randomize them into 3
groups, a control and 2 treatment groups. Is there a way I can compute
the number of classrooms needed for the study using the following

Desired power=0.8
Estimated effect size = 0.2
alpha = 0.0167
1 pre-test measure
1 post-test measure
approx correlation between the two measures = 0.25

Joe McCrary
Senior Research Associate
260 Peachtree St., Suite 2200
Atlanta, GA 30303

ph: 541-782-8626
fax: 404-393-3797

Skype: joe.mccrary
Google talk; joe.mccrary

"No problem can be solved from the same level of consciousness that created it."
--Albert Einstein
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