Hi
On Fri, 2 Mar 2001, John W. Kulig wrote:
> To combine 2 studies in a meta-analysis and test for significance, you
> need to get the Z value for the p reported. But the p must be
> one-tailed. So, if you have a two group study where t = 2.00 and p = .08
> two tailed, you'd half the p to .04. In this case the corresponding Z =
> 1.75.
>
> But what if the same result came from an F test? If the same numbers
> were run through a simple oneway ANOVA, F = 4.0 and p = .08. I have
> always halved the p value from an F test and treated it as a one-tailed
> test.
>
> The F distribution (as a statistical distribution) is one-tailed. But if
> null is true it's equally likely that mean1 > mean 2, or, mean1 < mean
> 2. The F distribution is used to capture both research outcomes into the
> right tail (or virtually all of it). If I specify _in advance_ that
> mean1 > mean2, can't we just half the p and say p = .04 one-tailed? I'm
> pretty sure a statistician explained this to me this way.
>
> Some people say "one-tailed" while others say "one-sided." I suspect
> that "tails" refers to the actual distribution of F, while "sided"
> refers to the possible research outcomes - and the two are separate
> concepts. But, if anyone thinks I'm off my rocker, please let me know.
Another term that is sometimes used is directional vs.
non-directional to refer to the alternative hypothesis. You can
also think of the F distribution as the t-test folded over so
that both the "negative" and "positive" tails are in the upper
end of the F distribution. In any case you are correct that the
p for F can be halved to get the one-tail probability. This
causes no end of consternation when teaching the use of tables,
because it means that you have to use area=.10 to do a
directional test with F when alpha=.05 (i.e., alpha = area/2).
Although you are correct on this issue, I am reluctant to say
definitively whether or not you are off your rocker in a more
general sense because (1) you don't provide enough information
about yourself, (2) I don't have your MMPI or PAI profile
available, and (3) I'm not a clinical psychologist
anyway. However, your postings over the years haven't shown any
obvious delusional signs.
Best wishes
Jim
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James M. Clark (204) 786-9757
Department of Psychology (204) 774-4134 Fax
University of Winnipeg 4L05D
Winnipeg, Manitoba R3B 2E9 [EMAIL PROTECTED]
CANADA http://www.uwinnipeg.ca/~clark
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