Sorry this is going to the general tips group rather than tips-methods, but I
haven't seen much on the methods list and don't know if that list is still active.
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?
--
* John W. Kulig, Department of Psychology ************************
* Plymouth State College Plymouth NH 03264 *
* [EMAIL PROTECTED] http://oz.plymouth.edu/~kulig *
*******************************************************************
* "Eat bread and salt and speak the truth" Russian proverb *
*******************************************************************
