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

[Michael Norman Mitchell <Michael.Norman.Mitchell@gmail.com>]

Greetings

  I concur with Scott, that Optimal Design is very useful in this kind 
of situation. I would especially recommend taking time with the user 
manual for this program. The program is super well designed and easy to 
use, and I think it is very tempting just to use the program without the 
documentation. However, I feel that is cheating oneself because the 
manual is exceptional in the way that it explains the underlying issues 
and how to conceptualize them in terms of using their software. I think 
the trickiest bit, in your case, will be the combination of the 
clustering and the repeated measures. I think you will get the most 
mileage out of conceptualizing the "pre test" as a level 1 covariate and 
I believe the optimal design software will permit this. However, as the 
manual describes, it is "level 2" covariates that will have the biggest 
impact on power. If the software does not support level 1 covariates in 
such a model, I don't think it will alter the power estimate that much.

I hope that is useful additional information.

Michael N. Mitchell
See the Stata tidbit of the week at...
http://www.MichaelNormanMitchell.com

On 2010-05-03 6.08 PM, Scott Baldwin wrote:
> I'm not aware of any built in or user-written method in Stata for
> computing power in cluster randomized trials. You can try the Optimal
> Design software
> (http://sitemaker.umich.edu/group-based/optimal_design_software) or
> the formulae in David Murray's book "Design and analysis of group
> randomized trials."
>
> Best,
> Scott
>
>
> On Mon, May 3, 2010 at 5:23 PM, Joe McCrary<joe.mccrary@gmail.com>  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
> > criteria:
> >
> > 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
> > WestEd-Atlanta
> > 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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