Posted by
Swank, Paul R on
May 04, 2011; 8:48pm
URL: http://spssx-discussion.165.s1.nabble.com/Dummy-coded-predictor-in-linear-regression-with-unequal-group-sizes-tp4370421p4371073.html
It would be 2/(1/20 + 1/180) = 36 per group. I verified that by actually calculating the power and by simulation.
Dr. Paul R. Swank,
Professor and Director of Research
Children's Learning Institute
University of Texas Health Science Center-Houston
-----Original Message-----
From: Rich Ulrich [mailto:
[hidden email]]
Sent: Wednesday, May 04, 2011 1:59 PM
To: Swank, Paul R;
[hidden email]
Subject: RE: Dummy-coded predictor in linear regression with unequal group sizes
In particular, the "effective Ns" for a two group comparison
can be computed as the reciprocal-mean: The reciprocal of
the average of the reciprocals.
For 10 vs 90 (deriving from the original Question), N=200,
this says that a comparison has the same power as comparing
two groups of 20 -
Reciprocals are .1 and .011; average of them is .055;
reciprocal of that is about 20. Effectively, two groups of 20.
--
Rich Ulrich
----------------------------------------
> Date: Wed, 4 May 2011 12:48:04 -0500
> From:
[hidden email]
> Subject: Re: Dummy-coded predictor in linear regression with unequal group sizes
> To:
[hidden email]
>
> It will reduce the power of you significance test. For example, with a simple t test and a total sample size of 94, the power to detect a .5 sd effect size is .626 when the group sizes are 20 vs 74 but .776 when they are 47 vs 47.
>
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