On 13 Mar 2001 07:12:33 -0800, [EMAIL PROTECTED] (dennis roberts) wrote:
>1. some test statistics are naturally (the way they work anyway) ONE sided
>with respect to retain/reject decisions
>
>example: chi square test for independence ... we reject ONLY when chi
>square is LARGER than some CV ... to put a CV at the lower end of the
>relevant chi square distribution makes no sense
>
Hmm... do not want to start flame war but just can not go by such HUGE
misconception about chi squared test. Indeed exactly reverse is true :
chi squred test is always two tailed. There is nothing to prove just
look at the definition : Khi^2(n)=sum(Z^2).
Altogether with many other answers I saw on sci.stat.* this makes grow
my desire to unsubscribe.
Now getting back to original question. If you declared to carry on one
tailed test and this was not significant your conclusion is simple as
follows: "We could not show that reaction time in condition A is
longer than in condition B.(full stop)". That is your main conclusion.
Now on the side you can play around to try to explain this (as in your
case it appears that the reason was in small subset). And conclude
that to show this you are going to start another study.
Finally on the subject of your message. My ansver is : ALWAYS DO TWO
TAILED TESTS. In a nutshell there are two major resons to do two
tailed tests. First your problem is a good example - you tested if A
was superior to B instead to test the difference and you failed.
Second imagine your test reached 5% barreer. In this case you will
probably give the reader mean difference with its confidence interval.
This CI is 95% and may contain 0. Seems weird isn't it?
Incidentally my opinion agrees with international harmonisation
guidelines. Just dig FDA site to find them. There are half-page
additional explanations why one tailed tests with 5% are unacceptable.
The result you can not submit a drug for approval based on studies
with one tailed 5% rate tests.
I am dermatologist not statistitian and all those questions seems
obvious to me. I am disappointed.
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