R help - Nov 2008 - About continuity correction option in the mantelhaen.test function.

Hello,
   I was using the mantelhaen.test function (2x2 J tables for
conditional independence testing). I noticed that the option for the
continuity correction (correction=T or correction=F) sometimes made a
difference while sometimes it did give the same results regardless of
correction=T or correction=F.

  Does the continuity correction apply only in certain cases or in
certain strata with some rules ?
  Could you let me know why I am seeing no difference sometimes
regardless of correction=T or F ?
  Thank you so much,
  Insu 

*************************************************
Insu Paek 
Educational Testing Service
Rosedale Road MS - 02P
Princeton, NJ 08541
(Tel) 609 - 734 - 1450 
Email: ipaek@ets.org 
*************************************************




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Paek, Insu wrote:
> Hello,
>    I was using the mantelhaen.test function (2x2 J tables for
> conditional independence testing). I noticed that the option for the
> continuity correction (correction=T or correction=F) sometimes made a
> difference while sometimes it did give the same results regardless of
> correction=T or correction=F.
> 
>   Does the continuity correction apply only in certain cases or in
> certain strata with some rules ?
>   Could you let me know why I am seeing no difference sometimes
> regardless of correction=T or F ?
An example would help...

According to the code, the continuity correction is turned off if DELTA 
< 0.5, so you should see it only in cases where the p-value is close to 
1. However, the output should say so:

             DELTA <- abs(sum(x[1, 1, ] - s.x[1, ] * s.y[1, ]/n))
             YATES <- ifelse(correct && (DELTA >= 0.5), 0.5, 0)
             STATISTIC <- ((DELTA - YATES)^2/sum(apply(rbind(s.x,
                 s.y), 2, prod)/(n^2 * (n - 1))))
....
             METHOD <- paste("Mantel-Haenszel chi-squared test",
                 ifelse(YATES, "with", "without"),
                               "continuity correction")

What worries me more is that it seems that 'alternative' is being 
ignored in the !exact case:


 >  mantelhaen.test(UCBAdmissions,alt="l")$p.value
[1] 0.2322635
 >  mantelhaen.test(UCBAdmissions)$p.value
[1] 0.2322635
 >  mantelhaen.test(UCBAdmissions,exact=T)$p.value
[1] 0.2277625
 >  mantelhaen.test(UCBAdmissions,exact=T, alt="l")$p.value
[1] 0.1159937



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