cvandy wrote:
When I use wilcox.test, I get vastly different p-values than the problems
from Statistics textbooks.
For example:
The following problem comes from "Applied Statistics and Probability for
Engineers", 2nd Edition, by D. C. Montgomery. Page736, problem 14.7. The
problem is to compare the sample data with a population median of 8.5. The
book answer is p = 0.25, wilcox.test answer is p = 0.573.
I've tried several other similar problems with similar results. I've copied
the following directly from my workspace.
wilcox.exact (from exactRankTests) gives
> wilcox.exact(x - 8.5)
Exact Wilcoxon signed rank test
data: x - 8.5
V = 80.5, p-value = 0.5748
so I'd suspect the textbook. One-sided p-value perhaps? or table
limitation (as in "p > .25"). If you want to dig deeper, you'll probably
have to check the computations implied by the text.
Thanks for any help,
CHV
x<-c(8.32,8.05,
8.93,8.65,8.25,8.46,8.52,8.35,8.36,8.41,8.42,8.30,8.71,8.75,8.6,8.83,8.5,8.38,8.29,8.46)
wilcox.test(x,y=NULL,mu=8.5)
Wilcoxon signed rank test with continuity correction
data: x
V = 80.5, p-value = 0.573
alternative hypothesis: true location is not equal to 8.5
Warning messages:
1: In wilcox.test.default(x, y = NULL, mu = 8.5) :
cannot compute exact p-value with ties
2: In wilcox.test.default(x, y = NULL, mu = 8.5) :
cannot compute exact p-value with zeroes
Charles H Van deZande
--
O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B
c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K
(*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalga...@biostat.ku.dk) FAX: (+45) 35327907
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