Posted by
Rich Ulrich on
May 04, 2011; 6:58pm
URL: http://spssx-discussion.165.s1.nabble.com/Dummy-coded-predictor-in-linear-regression-with-unequal-group-sizes-tp4370421p4370758.html
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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