[R] Problem applying McNemar's - Different values in SPSS and R
Johannes Huesing
johannes at huesing.name
Wed Dec 29 15:58:49 CET 2010
Marc Schwartz <marc_schwartz at me.com> [Wed, Dec 29, 2010 at 03:28:56PM CET]:
>
> On Dec 29, 2010, at 6:48 AM, Manoj Aravind wrote:
>
> > Thank you Marc :)
> > It Certainly helped me to get the exact value of P.
> > How to understand when to apply mcnemar.exact or just mcnemar.test?
[...]
>
> Generally speaking, exact tests are used for "small-ish" sample
sizes. Frequently when n <100 and in many cases, much lower (eg. <50
or <30). The methods tend to become computationally impractical on
"larger" data sets.
Sorry for chiming in again here, but binomial tests are computationally
cheap:
> system.time(binom.test(48000, 100000))
User System verstrichen
0.072 0.000 0.077
You are certainly correct on Fisher's Exact Test with larger tables
or Wilcoxon's Signed Rank test.
[...]
> One exception to the above comment, is the use of Fisher's Exact Test (FET), which is typically advocated by folks as an alternative to a chi-square test when **expected** cell counts are <5. However, much has been written in recent times relative to just how conservative the FET is. One resource is:
>
> http://www.iancampbell.co.uk/twobytwo/twobytwo.htm
>
That's only because people shy away from the randomized version :-)
--
Johannes Hüsing There is something fascinating about science.
One gets such wholesale returns of conjecture
mailto:johannes at huesing.name from such a trifling investment of fact.
http://derwisch.wikidot.com (Mark Twain, "Life on the Mississippi")
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