Date: Wed, 19 Sep 2007 18:09:00 +0200
Reply-To: Marta Garcia-Granero <mgarciagranero@gmail.com>
Sender: "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From: Marta Garcia-Granero <mgarciagranero@gmail.com>
Subject: Re: sample size calculation
In-Reply-To: <3CD837F073E6EC4AB2F84495C5AC1A38183CE0@W3C06PEVS.hdwa.health.wa.gov.au>
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Hi Hunna<br>
<br>
MorePower (see ref. below) is a great freeware utility for ANOVA
designs. Unfortunately, you don't give all the required details<br>
<br>
- Which effect are you interested in? (main effects, 2-way
interactions, three-way interactions...)<br>
- You mention unequal cells. How big the unbalance is? The sample size
needed can increase dramatically if the design is heavily unbalanced.<br>
<br>
Assuming that when you indicate "large effect" you are talking about a
partial eta-square of 0.14, MorePower output syas (for a <b>perfectly</b>
balanced design) that each cell should have at least 8 cases (since a
2x2x2 design has 8 cells, the total sample size required is: Nt=64).
This is the sample size needed for the 3-way interaction. Figures for
main effects and 2-way interactions were slightly lower (total sample
sizes needed Nt=60 and Nt=56, respectively).<br>
<br>
MorePower doesn't compute sample sizes for unbalanced designs. If you
are going to have unequal cells, make sure that the <b>harmonic</b>
mean of your sample sizes is equal to the one given by the program (for
each cell).<br>
<br>
Anyway: is a large effect expected (based on your previous experience)
or should you consider the possibility of finding smaller effects?<br>
<br>
Program reference:<br>
<i>"MorePower by <a class="moz-txt-link-abbreviated" href="mailto:Jamie.Campbell@USask.Ca">Jamie.Campbell@USask.Ca</a>,<br>
Dept. of Psychology, University of Saskatchewan, October 31, 2006.<br>
Implementation documentation and details about the built-in examples
at: <a class="moz-txt-link-freetext" href="http://">http://</a><br>
duke.usask.ca/~campbelj/work/MorePower.html<br>
If you use the calculator, please cite Campbell, J.I.D. & Thompson,
V.A. (2002). More power to you: Simple power calculations for treatment
effects with one degree of freedom. Behavior Research Methods,
Instruments, and Computers, 34, 332-337."</i><br>
<br>
HTH,<br>
Marta Garcia-Granero<br>
<br>
<blockquote
cite="mid:3CD837F073E6EC4AB2F84495C5AC1A38183CE0@W3C06PEVS.hdwa.health.wa.gov.au"
type="cite">
<pre wrap="">Anyone quick at calculating required sample sizes? Annoying government
worksite restrictions are blocking me from downloading power software
like G*Power.
I am after the required sample size for detecting a -large effect at 80%
power and alpha level of .05 for a 2x2x2 ANOVA with unequal cells. Can
anyone give me the required sample size and indicate which program they
used so I have a reference?</pre>
</blockquote>
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