John Sorkin wrote:
> Timothy,
> I believe you are mistaken. Fisher's exact test give the correct answer
> even in the face of small expected values for the cell counts. Pearson's
> Chi-square approximates Fisher's exact test and can give the wrong
> answer when expected cell counts are low. Chi-square was developed
> because it is computationally "simple". Fisher's exact test,
> particularly with tables larger than 2 by 2 can be computationally
> complex. The value of the Chi-square statistic becomes closer and closer
> to Fisher's exact test as the expected cell counts become larger.
> John
John,
I'll have to disagree a bit. Pearson's can still work with low expected
frequencies. It was not intended to approximate Fisher's test. And
Fisher's is conservative. The Pearson chi-square with Yates' continuity
correction was intended to approximate the more conservative Fisher test.
Cheers,
Frank
>
> John Sorkin M.D., Ph.D.
> Chief, Biostatistics and Informatics
> Baltimore VA Medical Center GRECC and
> University of Maryland School of Medicine Claude Pepper OAIC
>
> University of Maryland School of Medicine
> Division of Gerontology
> Baltimore VA Medical Center
> 10 North Greene Street
> GRECC (BT/18/GR)
> Baltimore, MD 21201-1524
>
> 410-605-7119
> -- NOTE NEW EMAIL ADDRESS:
> [EMAIL PROTECTED]
>
>
Timothy Mak <[EMAIL PROTECTED]> 9/12/2005 11:45:28 AM >>>
>
>
> I have heard that people favour the Pearson's Chi-square over Fisher's
>
> exact test because the latter is more conservative. Some people
> therefore
> only use Fisher's exact test when some of the expected counts are too
> small. But nowadays we can quite easily calculate the exact p-value
> based
> on the Pearson statistic, provided it's not a huge table (SPSS can do
> it).
> Is there any place for Fisher's exact test then?
>
> Tim
>
>
> [[alternative HTML version deleted]]
>
> __
> R-help@stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
>
--
Frank E Harrell Jr Professor and Chair School of Medicine
Department of Biostatistics Vanderbilt University
__
R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
