Posted by
Rich Ulrich on
Apr 17, 2011; 5:05am
URL: http://spssx-discussion.165.s1.nabble.com/Questions-about-ANOVAs-and-Sphericity-tp4297706p4308472.html
>
> When you run a post-hoc test in ANOVA and are interested in a
> one-tailed test it posible to use the reported sig. as sig./2 as a p
> value for a one tailed test?
>
As Paul S. says, so long as the result is in the proper
direction, that is the way you would compute it.
However, you should keep in mind that some audiences and
many journals are hostile to one-tailed tests. There should
be a *strong* expectation, a-priori, that the result will be
in that direction. The one time that I remember reporting
a one-tailed result is one where we had written the test into
our grant proposal, years before, justifying its use on this
secondary hypothesis by the predicted direction *and* the
lack of power that we expected.
ALSO.
There is a theoretical possibility for two-tailed tests where
tails are not the same size. - In statistical theory, these are
introduced when teaching about the concept of a test that is "UMP",
or "Uniformly Most Powerful." A test that is UMP has more "power"
to reject the null than any other test of the same hypothesis.
A one-tailed t-test is the UMP test for a difference of means
between (say) normal samples.
Only a one-tailed test will ever be UMP. A two-tailed t-test
with equal-sized tails will have less power for one set of the
alternatives (A > B, or B > A) than a t-test with unequal tails.
And so on, generally.
It is a very useful convention which says that we assign half
the p-value to each side. But would you consider it fair, for
your variable, to take 0.001 as the cut-off for a test in one
direction, and 0.049 as the cut-off in the other direction?
- That would mean that you would be happy to treat a test result
that is 0.01 (say) in the wrong direction as a possibly curious
and suggestive, but insubstantial, result. - I ask myself this
question, when probing my own feelings about a particular test.
--
Rich Ulrich
=====================
To manage your subscription to SPSSX-L, send a message to
[hidden email] (not to SPSSX-L), with no body text except the
command. To leave the list, send the command
SIGNOFF SPSSX-L
For a list of commands to manage subscriptions, send the command
INFO REFCARD