From: dih69530@syd.odn.ne.jp 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 ，chi-sq must be 0

From: P Ehlers <ehlers@math.ucalgary.ca> dih69530@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 ，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. Peter Ehlers

From: Prof Brian Ripley <ripley@stats.ox.ac.uk> On Sun, 30 Oct 2005, P Ehlers wrote: > dih69530@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 ，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@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

From: P Ehlers <ehlers@math.ucalgary.ca> Prof Brian Ripley wrote: > On Sun, 30 Oct 2005, P Ehlers wrote: > >> dih69530@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 ，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

NOTES: What R does is standard, if not necessarily optimal

Audit (from Jitterbug): Thu Nov 3 20:22:08 2005 thomas changed notes Thu Nov 3 20:22:18 2005 thomas moved from incoming to Analyses