Hi
On Tue, 6 Feb 2001, Thom Baguley wrote:
> Donald Burrill wrote:
> > Well, it _might_ be. Depends on what hypothesis was being tested,
> > doesn't it? And so far "rjkim" hasn't deigned to tell us that.
>
> Yes, though I think the vocabulary can obscure what goes on. To me a
> "one-tailed" test should refer to the distribution to retain the meaning of
> "tail" and hence is a confusing term if used without further explanation.
The problem is that one-tailed test is taken as synonymous with
directional hypothesis (e.g., Ha: Mu1>Mu2). This causes no
confusion with distributions such as the t-test, because
directional implies one-tailed. This correspondence does not
hold for other statistics, such as the F and Chi2. One can get a
large F by either Mu1>Mu2 or Mu1<Mu2 (or by positive or negative
R, ...). Therefore the one-tail of the distribution corresponds
(normally) to a two-tailed or non-directional test. However,
there is absolutely nothing wrong with making the necessary
adjustment to make the test directional (i.e., equivalent to the
one-tailed t-test), and therefore referring to it (confusingly,
of course) as a one-tailed test. To make F directional, one
simply halves p from the statistical output or looks up the
critical value of F with 2*alpha (e.g., .10). The same would
hold for Chi2 and is presumably what happened with the paper
referred to initially (assuming knowledge of statistics). That
is, the Chi2 under many applications would be insensitive as to
the direction by which observed values differed from expected
values, making it a non-directional/two-tailed test without some
adjustment. But such adjustment would be appropriate if the
direction of differences was predicted, just as for the F.
Best wishes
Jim
============================================================================
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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