I have referred to this procedure as being a "half-tailed test." The
one-tailed probability for this application of F (and for the typical
applications of Chi-Square as well) are, indeed, nondirectional, since the
numerator is representing the effect as a squared quantity. Consider your
guessing the direction of effect as event A and your getting a sample as
discrepent with a nondirectional null as event B. These events should be
independent, so, applying the multiplication rule under independence, the
directional p is .5*P(B). Taking this a bit further, if you had three
groups and could convince us that you predicted the ordering of the means of
the three, a one-sixth tailed probability would be appropriate.
----- Original Message -----
From: "jim clark" <[EMAIL PROTECTED]>
To: "TIPS" <[EMAIL PROTECTED]>
Sent: Friday, March 02, 2001 8:33 PM
Subject: Re: Question about one-tailed tests
> 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
>
>
============================================================================
> 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
>
============================================================================
>
