mann-whitney test with more groups

Dear r-helpers,
I want to test groups of samples for significant differences.
Question: Does Group1 differ significantly from group2.
This is a question to be answered by mann-whitney-u-test.

I know that I can use wilcox.test with 2 samples.

My problem: How can r perform the test automatically if there are more than 2 groups in my data frame.
Test group1 vs. 2, 1 vs. 3, 1 vs. 4, etc.


This is my skript:
Deckung <- read.table("Gesamtdeckung.csv", sep=";", header=TRUE, dec=",", row.names=1)

x <- Deckung$Gesamtdeckung
y <- Deckung$Klasse

#U-Test
wilcox.test(x ~ y, paired = FALSE, subset = y %in% c("1", "2"))
wilcox.test(x ~ y, paired = FALSE, subset = y %in% c("1", "3"))
wilcox.test(x ~ y, paired = FALSE, subset = y %in% c("2", "3"))

Any help would be greatly appreciated!

Thanks
Kim 

Kim Vanselow wrote:

Dear r-helpers,
I want to test groups of samples for significant differences.
Question: Does Group1 differ significantly from group2.
This is a question to be answered by mann-whitney-u-test.

I know that I can use wilcox.test with 2 samples.

My problem: How can r perform the test automatically if there are more 
than 2 groups in my data frame.
Test group1 vs. 2, 1 vs. 3, 1 vs. 4, etc.


This is my skript:
Deckung <- read.table("Gesamtdeckung.csv", sep=";", header=TRUE, 
dec=",", row.names=1)

x <- Deckung$Gesamtdeckung
y <- Deckung$Klasse

#U-Test
wilcox.test(x ~ y, paired = FALSE, subset = y %in% c("1", "2"))
wilcox.test(x ~ y, paired = FALSE, subset = y %in% c("1", "3"))
wilcox.test(x ~ y, paired = FALSE, subset = y %in% c("2", "3"))

Any help would be greatly appreciated!

Thanks
Kim 

This sounds like serious data dredging, but if you're
sure that it's what you want, try the combn() function:

 y <- gl(4, 5)
 x <- rnorm(20)
 m <- cbind(t(combn(4, 2)), NA)
 for(i in 1:nrow(idx))
    m[i, 3] <-
      wilcox.test(x ~ y, subset = y %in% idx[i,])$p.value
 m

 -Peter Ehlers

Sorry, correction below.

Peter Ehlers wrote:

Kim Vanselow wrote:

Dear r-helpers,
I want to test groups of samples for significant differences.
Question: Does Group1 differ significantly from group2.
This is a question to be answered by mann-whitney-u-test.

I know that I can use wilcox.test with 2 samples.

My problem: How can r perform the test automatically if there are 
more than 2 groups in my data frame.
Test group1 vs. 2, 1 vs. 3, 1 vs. 4, etc.


This is my skript:
Deckung <- read.table("Gesamtdeckung.csv", sep=";", header=TRUE, 
dec=",", row.names=1)

x <- Deckung$Gesamtdeckung
y <- Deckung$Klasse

#U-Test
wilcox.test(x ~ y, paired = FALSE, subset = y %in% c("1", "2"))
wilcox.test(x ~ y, paired = FALSE, subset = y %in% c("1", "3"))
wilcox.test(x ~ y, paired = FALSE, subset = y %in% c("2", "3"))

Any help would be greatly appreciated!

Thanks
Kim 

This sounds like serious data dredging, but if you're
sure that it's what you want, try the combn() function:

 y <- gl(4, 5)
 x <- rnorm(20)
 m <- cbind(t(combn(4, 2)), NA)
 for(i in 1:nrow(idx))
    m[i, 3] <-
      wilcox.test(x ~ y, subset = y %in% idx[i,])$p.value
 m

 -Peter Ehlers

  y <- gl(4, 5)
  x <- rnorm(20)
  m <- cbind(t(combn(4, 2)), NA)
  for(i in 1:nrow(m))  # change 'idx' to 'm'
     m[i, 3] <-
       wilcox.test(x ~ y, subset = y %in% m[i,])$p.value  # ditto