Hi
On Wed, 27 Jan 1999, John W. Kulig wrote:
> You have a simple two-group (independent) situation, and do an significance
> test contrasting the two means (t or F). You have several other studies doing
> the same thing. To combine them into one overall test for significance,
> Rosenthal and Rosnow (1991) recommend getting Z scores from the p levels
> reported in each study. But, you have to use the _one-tail_ probabilities for
> the meta-analysis. This is easy if t was reported. If t was reported as
> two-tailed, just half the p reported. So if they report t = such-and-such and
> two-tailed p = .08, .04 is the one-tailed value. But what about F? The F
> distribution is naturally one-tailed (well, there is a little area to the left
> which might represent the "true good to be true" reject, but that is of little
> significance). It would be improper to "half" the reported p value from ANOVA,
> but, isn't it also the case that if we simply changed the reported F to a t
> (since F = t^2), and _then_ half the p to represent only one-tail, we are
> doing the same thing? If that's the case, then is it true (for practical
> purposes only) that we can simply half the reported p from F and consider it
> as if it were a one-tailed directional test, in those cases where only two
> groups were used? After all, under Null where mean 1 = mean 2, half of the
> cases in the rejection area of the F distribution will be when mean 1 > mean
> 2, and the other half when mean 1 < mean 2. Or am I missing something?
Halving the p-value for F is exactly what you should do.
Although the term used is one-tailed and two-tailed, the critical
factor is not whether the rejection region has two tails (as in
t) or one tailed (as in F), but whether the statistic is
sensitive to the direction of difference or r. Think of F as a
folded over t and you will see that F can produce rejection in
two ways, if A>B or A<B; hence it is a two-tailed test.
Alternatively, SS treatment or SS regression in F is insensitive
to direction of difference or r.
Best wishes
Jim
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James M. Clark (204) 786-9313
Department of Psychology (204) 774-4134 Fax
University of Winnipeg 4L02A
Winnipeg, Manitoba R3B 2E9 [EMAIL PROTECTED]
CANADA http://www.uwinnipeg.ca/~clark
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