[R] The three routines in R that calculate the wilcoxon signed-rank test give different p-values.......which is correct?

From: Michael G Rupert <mgrupert_at_usgs.gov>
Date: Tue, 12 Apr 2011 17:57:13 -0600


I have a question concerning the Wilcoxon signed-rank test, and specifically, which R subroutine I should use for my particular dataset. There are three different commands in R (that I'm aware of) that calculate the Wilcoxon signed-rank test; wilcox.test, wilcox.exact, and wilcoxsign_test. When I run the three commands on the same dataset, I get different p-values. I'm hoping that someone can give me guidance on the strengths and weaknesses of each command, why they produce different p-values, and which one is the most appropriate for my particular needs.

First, let me describe the dataset I am working with. The project I am working on collected water samples from groups/networks of about 30 water wells and analyzed them for nitrate, major ions, and other chemical constituents. We revisited those same wells about 10 years later and analyzed the water samples for the same chemical constituents. I now have a paired dataset, and the question I would like to answer is whether there was a "significant" change in concentrations of those chemical constituents (such as nitrate or chloride). Concentrations measured in water from some wells have increased, some have decreased, and some have stayed the same over the ten-year time period. In water from some wells, the concentrations were below the laboratory detection limits, so those concentrations are "tied" at the reporting level. The following is an example of the data I am evaluating.

x <- c(13.60, 9.10, 22.01, 9.08, 1.97, 2.81, 0.66, 0.97, 0.21, 2.23, 0.08, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 3.44, 15.18, 5.25, 4.27, 17.81) y <- c( 4.32, 3.39, 16.36, 7.10, 0.08, 2.02, 0.19, 0.59, 0.06, 2.15, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 4.02, 16.13, 7.30, 7.98, 24.37)

The nonparametric Wilcoxon signed-rank test seems to be the most appropriate test for these data. There are two different methods to calculate the signed-rank test. The first is by Wilcoxon (1945), who discards any tied data and then calculates the signed ranks. The second method incorporates tied values in the ranking procedure (see J.W. Pratt, 1959, Remarks on zeros and ties in the Wilcoxon signed rank procedure: Journal of the American Statistical Association, Vol. 54, No. 287, pp. 655-667). There are two commands in R that calculate the original method by Wilcoxon (that I know of), wilcox.test and wilcoxsign_test (make sure to include the argument "zero.method = c("Wilcoxon")"). There are two other commands in R that incorporate ties in the signed-rank test, wilcox.exact and wilcoxsign_test (make sure to include the argument"zero.method = c("Pratt")").

Here's my problem. I get different p-values from each of the 4 signed-rank tests in R, and I don't know which one to believe. Wilcox.test and wilcoxsign_test(zero.method = c("Wilcoxon") calculate the standard Wilcoxon signed-rank test. Even though they are not designed to deal with tied data, they should at least calculate the same p-value, but they do not. I ran the same datasets in SYSTAT and Minitab to check on the results from R. Minitab gives the same results as wilcox.test, and SYSTAT gives the same results as wilcoxsign_test(zero.method = c("Wilcoxon"). Similarly, wilcox.exact and wilcoxsign_test(zero.method = c("Pratt")) are designed to incorporate ties, but they give different p-values from each other. The signed-rank test procedure is relatively straightforward, so I'm surprised I'm not getting identical results.

To check on these R commands, I calculated the signed-rank tests using the dataset shown on page 658-659 of Pratt (1959). These R routines do not produce the same results as that listed in Pratt, which makes me think that the R routines are not calculating the statistics correctly. The following text shows the commands I use in R to calculate the signed-rank test using these different R commands:

Thanks in advance for any assistance.

--Mike

#################################################################################
library(exactRankTests) #this loads the package for calculating the modified signed-rank test
library(coin) #this adds additional routines for the wilcoxon signed-rank test and the Pratt signed-rank test
#
# Data from Page 658 of Pratt

x <- c(1, 1, 1, 1, 1, 7, 10, 12, 13, 16, 17) y <- c(1, 1, 3, 4, 6, 1, 1, 1, 1, 1, 1)
#
# STANDARD WILCOXON SIGNED RANK USING WILCOX.TEST
wilcox.test(x, y, alternative='two.sided', paired=TRUE)
# STANDARD WILCOXON SIGNED-RANK USING WILCOXSIGN_TEST.
wilcoxsign_test (x ~ y, zero.method = c("Wilcoxon"))
#
#
# MODIFIED WILCOXON SIGNED-RANK USING WILCOX.EXACT
wilcox.exact(x, y, alternative='two.sided', paired=TRUE, mu=0)
# PRATT SIGNED-RANK TEST

wilcoxsign_test (x ~ y, zero.method = c("Pratt"))

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