From ehlers at math.ucalgary.ca  Mon Oct 31 17:12:13 2005
From: ehlers at math.ucalgary.ca (ehlers@math.ucalgary.ca)
Date: Mon, 31 Oct 2005 17:12:13 +0100 (CET)
Subject: [Rd] Yates' correction for continuity in chisq.test (PR#8265)
Message-ID: <20051031161213.BAA1022151@slim.kubism.ku.dk>



Prof Brian Ripley wrote:

> 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.
> 

You're quite right that, for consistency, the correction should be
applied even in the silly example I gave. And, of course, one
should not be doing a chisquare test on silly examples.

Peter Ehlers



[Rd] Yates' correction for continuity in chisq.test (PR#8265)
ehlers@math.ucalgary.ca