I'm going to retract my previous statement and agree with the Stata
manual that the chi square and exact RxC tests for independence
reported in Stata are properly called one-sided.
I'll again use the analogy of the chi square Test, because I believe
that at least one version of the exact test ranks tables on the value
of their chi square statistic. The chi square test is a test of fit
of the model of independence, and rejects if the chi square statistic
is "too big". Call this a "right-tail" test. The implicit parameter
here is the sum of squares in which counts are replaced by
probabilities.
However one can conceive of a test of independence, in which the
alternative is 'too good a fit". For example, RA Fisher believed that
some of Gregory Mendel's observations were too close to expectation to
have occurred by chance. (For a revisionist view see: CE Novitski
(2004) Revision of Fisher's Analysis of Mendel's Garden Pea
Experiments. Genetics 166: 1139-1140
http://www.genetics.org/cgi/content/full/166/3/1139 ).
If one conducted a test with this alternative, it would reject if the
Chi Square statistic is "too small". The analogous exact test would
do the same. This would be a "left-tail" test.
The chi square and exact tests for independence reported by Stata are
indeed the right-tail tests and so are "one-sided".
I apologize to Tiago for my misleading comments.
-Steve
On Behalf Of Tiago V. Pereira
>> Sent: Friday, May 01, 2009 5:39 AM
>> To: [email protected]
>> Subject: st: [iso-8859-1] Fisher´s exact test for rxc tables: one-tailed or
>> two-ta iled[iso-8859-1] ?
>>
>> Just would like to thank David, Richard and Steve for their comments on
>> my last query. The two-sided option seems more plausible, indeed.
>>
>> Cheers!
>>
>> Tiago
>>
>>
>>
>>
>> ----------------------------------------------
>> Dear statalisters,
>>
>> I was reading some old post regarding exact tests in Stata and have found
>> the following message:
>>
>> http://www.stata.com/statalist/archive/2005-06/msg00029.html
>>
>> The author of this note comments on the possibility of the Fisher´s exact
>> test for rxc tables available in Stata to be one-tailed. Hence, is it two-
>> or one-tailed?
>>
>> Thanks in advance.
>>
>> All the best,
>>
>> Tiago
>>
*
* For searches and help try:
* http://www.stata.com/help.cgi?search
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/
