[R] Mann-Whitney U test discrepancies
Natalie O'Toole
notoole at mtroyal.ca
Wed Aug 15 21:24:24 CEST 2007
Hi,
I do want to use the Mann-Whitney test which ranks my data and then
uses
those ranks rather than the actual data.
Here is the R code i am using:
group1<-
c(1.34,1.47,1.48,1.49,1.62,1.67,1.7,1.7,1.7,1.73,1.81,1.84,1.9,1.96,2,
2,2.19,2.29,2.29,2.41,2.41,2.46,2.5,2.6,2.8,2.8,3.07,3.3)
> group2<-
c(0.98,1.18,1.25,1.33,1.38,1.4,1.49,1.57,1.72,1.75,1.8,1.82,1.86,1.9,1
.97,2.04,2.14,2.18,2.49,2.5,2.55,2.57,2.64,2.73,2.77,2.9,2.94,NA)
> result <- wilcox.test(group1, group2, paired=FALSE, conf.level =
0.95,
na.action)
paired = FALSE so that the Wilcoxon rank sum test which is equivalent
to
the Mann-Whitney test is used (my samples are NOT paired).
conf.level = 0.95 to specify the confidence level
na.action is used because i have a NA value (i suspect i am not using
na.action in the correct manner)
When i use this code i get the following error message:
Error in arg == choices : comparison (1) is possible only for atomic
and
list types
When i use this code:
group1<-
c(1.34,1.47,1.48,1.49,1.62,1.67,1.7,1.7,1.7,1.73,1.81,1.84,1.9,1.96,2,
2,2.19,2.29,2.29,2.41,2.41,2.46,2.5,2.6,2.8,2.8,3.07,3.3)
> group2<-
c(0.98,1.18,1.25,1.33,1.38,1.4,1.49,1.57,1.72,1.75,1.8,1.82,1.86,1.9,1
.97,2.04,2.14,2.18,2.49,2.5,2.55,2.57,2.64,2.73,2.77,2.9,2.94,NA)
> result <- wilcox.test(group1, group2, paired=FALSE, conf.level =
0.95)
I get the following result:
Wilcoxon rank sum test with continuity correction
data: group1 and group2
W = 405.5, p-value = 0.6494
alternative hypothesis: true location shift is not equal to 0
Warning message:
cannot compute exact p-value with ties in: wilcox.test.default(group1,
group2, paired = FALSE, conf.level = 0.95)
The W value here is 405.5 with a p-value of 0.6494
in SPSS, i am ranking my data and then performing a Mann-Whitney U by
selecting analyze - non-parametric tests - 2 independent samples and
then
checking off the Mann-Whitney U test.
For the Mann-Whitney test in SPSS i am gettting the following results:
Mann-Whitney U = 350.5
2- tailed p value = 0.643
I think maybe the descrepancy has to do with the specification of the
NA
values in R, but i'm not sure.
If anyone has any suggestions, please let me know!
I hope i have provided enough information to convey my problem.
Thank-you,
Nat
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