p-values

pperm seems reasonable, though I have not looked at the details.

	  We should be careful about terminology, however.  So-called "exact 
p-values" are generally p-values computed assuming a distribution over a 
finite set of possible outcomes assuming some constraints to make the 
outcome space finite.  For example, Fisher's exact test for a 2x2 table 
assumes the marginals are fixed.

	  I don't remember the details now, but I believe there is literature 
claiming that this may not be the best thing to do when, for example, 
when it is reasonable to assume that the number in each cell is Poisson. 
  In such cases, you may lose statistical power by conditioning on the 
marginals.  I hope someone else will enlighten us both, because I'm not 
current on the literature in this area.

	  The situation with "exact tests" and "exact p-values" is not nearly 
as bad as with so-called ""exact confidence intervals", which promise to 
deliver at least the indicated coverage probability.  With discrete 
distributions, it is known that 'Approximate is better than "exact' for 
interval estimation of binomial proportions', as noted in an article of 
this title by A. Agresti and B. A. Coull (1998) American Statistician, 
52:  119-126.  (For more on this particular issue, see Brown, Cai and 
Dasgupta 2003 "Interval Estimation in Exponential Families", Statistica 
Sinica 13:  19-49).

	  If this does not answer your question adequately, may I suggest you 
try the posting guide.  People report having found answers to difficult 
questions in the process of preparing a question following that guide, 
and when they do post a question, they are much more likely to get a 
useful reply.

	  spencer graves

p-values

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