[R] Mann-Whitney U test discrepancies

[Natalie O'Toole]

   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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