"Jason Osborne, Ph.D." wrote:
> I am working on a power analysis project- we are reviewing old journal
> articles to calculate observed effect sizes and power. Some of these
> articles, for example reporting t-test results, only give means and
> t-test, no standard deviation. thus, no effect size calculation is
> possible. I was hoping to estimate an effect size by converting a t to
What about this which does not require the sd?
d = t * sqrt( (n1 + n2) / (n1 * n2) )
Cohen (1988, p. 67, formula 2.5.3)
> an r. I seem to remember a formula that relates the two, but am having
> a dickens of a time tracking one down. The one I did track down, for
> calculating t from r, is not that helpful:
>
> t= r * sqrt(n-2)
> -------------
> sqrt(1-r^2)
>
> I want to be able to calculate r from t. I tried algebraically
> manipulating the formula, but never quite got it to where I could do
> this. Any advice?
r = sqrt ( t^2 / (t^2 + df) )
Rosenthal (1991, p. 19, formula 2.16)
But this is not a very good estimate if there is a big difference in the sizes of
the two groups compared. You can confirm that by calculating it both ways for an
unbalanced case and then converting both effect sizes to r.
r = d / sqrt( d^2 + 4)
Rosenthal (1991, p. 20, formula 2.20)
Hope this helps,
Chuck
----------------------------------------------
Chuck Cleland
Institute for the Study of Child Development
UMDNJ-Robert Wood Johnson Medical School
97 Paterson Street
New Brunswick, NJ 08903
phone: (732) 235-7699
fax: (732) 235-6189
http://www2.umdnj.edu/iscdweb/
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