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
__________________


Natalie,

It's best to provide at least a sample of your data.  Your field names 
suggest 
that your data might be collected in units of mm^2 or some similar 
measurement of area.  Why do you want to use Mann-Whitney, which will rank 

your data and then use those ranks rather than your actual data?  Unless 
your 
sample is quite small, why not use a two sample t-test?  Also,are your 
samples paired?  If they aren't, did you use the "paired = FALSE" option?

JWDougherty

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and provide commented, minimal, self-contained, reproducible code.



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