The three routines in R that calculate the wilcoxon signed-rank test give different p-values.......which is correct?
On 2011-04-12 16:57, Michael G Rupert wrote:
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.
Ahem .... that's a pretty strong claim.
Actually, the problem is user misunderstanding and the relevant
help pages do tell you where the differences lie.
Let's take the 3 functions one at a time, using your
x,y data from Pratt:
1. wilcox.test() in the stats package
This function automatically switches to using a Normal
approximation when there are ties in the data:
wilcox.test(x, y, paired=TRUE)$p.value
#[1] 0.05802402
(You can suppress the warning (due to ties) by specifying
the argument 'exact=FALSE'.)
This function also uses a continuity correction unless
told not to:
wilcox.test(x, y, paired=TRUE, correct=FALSE)$p.value
#[1] 0.05061243
2. wilcox.exact() in pkg exactRankTests
This function can handle ties (using the "Wilcoxon" method)
with an 'exact' calculation:
wilcox.exact(x, y, paired=TRUE)$p.value
#[1] 0.0546875
If you want the Normal approximation:
wilcox.exact(x, y, paired=TRUE, exact=FALSE)$p.value
#[1] 0.05061243 <-- cf. above
3. wilcoxsign_test() in pkg coin
This is the most comprehensive of these functions.
It is also the only one that offers the "Pratt" method
of handling ties. It will default to this method and
a Normal approximation:
pvalue(wilcoxsign_test(x ~ y))
#[1] 0.08143996
pvalue(wilcoxsign_test(x ~ y, zero.method="Pratt",
distribution="asympt"))
#[1] 0.08143996
You can get the results from wilcox.exact() with
pvalue(wilcoxsign_test(x ~ y, zero.method="Wilcoxon",
distribution="asympt"))
#[1] 0.05061243
and
pvalue(wilcoxsign_test(x ~ y, zero.method="Wilcoxon",
dist="exact"))
#[1] 0.0546875
As to which method you should use, that's up to you.
Peter Ehlers
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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