On 22/03/07, Kathryn Jane Gardner <[log in to unmask]> wrote:
> Thanks Jeremy for answering my questions. Just to clarify though, I was
> using directional to refer to 1 tailed tests (slip in terminology as I
> realise that these aren't necessarily the same thing, though they are
> often used synonymously). Someone in my dept said that if you run a 1
> tailed test (say a t test) and the means in are in the wrong direction,
> then the t test shouldn't be run i.e., you inspect the group means first
> and then only run t tests if results are in the direction you predicted.
> I think approach is consistent with what you were saying about not
> reporting a sig result if it is in the wrong direction. I think?
>
That's true, but if the means are in the wrong direction, would you
*really* say that you have found nothing.
Let's say that you do a test of intelligence on black and white
children. All the evidence (that I know of) would suggest that, if
you find a difference, it would be that the black children should
score lower.
So you run the test, and you find that the black children score
significantly higher. Do you then say "Well, that's a null result. I
found no effect."?
> I do see your point re: 1 tailed tests, and you clearly don't see a lot
> of them in the papers you review. You said "You can make a directional
> prediction based on anything. But if you then use that directional
> prediction to argue that you can do a one tailed test, then that's (in
> my opinion) naughty." I think like many, I have assumed that a 1 tailed
> test is used when a directional prediction is made and there is enough
> theory and/or evidence to do so. But it seems you don't agree with this
> and do not advocate using 1 tailed tests. As I said earlier, I haven't
> come across the use of 2 tailed tests for directional predictions. Maybe
> I am missing the basic underlying principles of the use of 1 and 2
> tailed tests and how they differ from directional and non-directional
> tests, but if am then so are many of my colleagues! So...if you could
> define the conditions for a 1 tailed test to be run, what would they
> be?
>
A one tailed test should be used when an effect in the opposite
direction to that which was expected would theoretically equivalent to
a zero effect.
Jeremy
P.S. Do you know Patrick McGhee, at UCLAN? He was my PhD supervisor.
--
Jeremy Miles
Learning statistics blog: www.jeremymiles.co.uk/learningstats
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