From ripley at stats.ox.ac.uk  Mon Oct 31 14:02:28 2005
From: ripley at stats.ox.ac.uk (ripley@stats.ox.ac.uk)
Date: Mon, 31 Oct 2005 14:02:28 +0100 (CET)
Subject: [Rd] Yates' correction for continuity in chisq.test (PR#8265)
Message-ID: <20051031130228.661A223895@slim.kubism.ku.dk>

On Sun, 30 Oct 2005, P Ehlers wrote:

> dih69530 at syd.odn.ne.jp wrote:
>> Full_Name: foo ba baz
>> Version: R2.2.0
>> OS: Mac OS X (10.4)
>> Submission from: (NULL) (219.66.32.183)
>>
>>
>> chisq.test(matrix(c(9,10,9,11),2,2))
>>
>> Chi-square value must be 0, and, P value must be 0
>> R does over correction
>>
>> when | a d - b c | < n / 2 &#65292;chi-sq must be 0
>
> (Presumably, you mean P-value = 1.)
> If you don't want the correction, set correct=FALSE. (The
> results won't differ much.)
>
> A better example is
>
>  chisq.test(matrix(c(9,10,9,10),2,2))
>
> for which R probably should return X-squared = 0.

R is using the correction that almost all the sources I looked at suggest. 
You can't go around adjusting X^2 for just some values of the data: the 
claim is that the adjusted statistic has a more accurate chisq 
distribution under the null.

I think at this remove it does not matter what Yates' suggested (although 
if I were writing a textbook I would find out), especially as the R 
documentation does not mention Yates.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595



[Rd] Yates' correction for continuity in chisq.test (PR#8265)
ripley@stats.ox.ac.uk