wilcoxon_test {r2spss}

Wilcoxon Signed Rank and Rank Sum Tests

Description

Perform a Wilcoxon signed rank test for a paired sample or a Wilcoxon rank sum test for independent samples on variables of a data set. The output is printed as a LaTeX table that mimics the look of SPSS output.

Usage

wilcoxon_test(data, variables, group = NULL, exact = FALSE)

## S3 method for class 'wilcoxon_test_SPSS'
to_SPSS(
  object,
  statistics = c("test", "ranks"),
  version = r2spss_options$get("version"),
  digits = NULL,
  ...
)

## S3 method for class 'wilcoxon_test_SPSS'
print(
  x,
  statistics = c("ranks", "test"),
  version = r2spss_options$get("version"),
  digits = 2:3,
  ...
)

wilcoxonTest(data, variables, group = NULL, exact = FALSE)

Arguments

data

a data frame containing the variables.

variables

a character vector specifying numeric variable(s) to be used. If group is NULL, the Wilcoxon signed rank test is performed and this should be a character vector specifying two numeric variables which contain the paired observations. If a grouping variable is specified in group, the Wilcoxon rank sum test is performed and this should be a character string specifying the numeric variable of interest.

group

a character string specifying a grouping variable for the Wilcoxon rank sum test, or NULL.

exact

a logical indicating whether the Wilcoxon rank sum test should also return the p-value of the exact test. The default is FALSE. Note that the p-value of the asymptotic test is always returned.

object, x

an object of class "wilcoxon_test_SPSS" as returned by function wilcoxon_test.

statistics

a character string or vector specifying which SPSS tables to produce. Available options are "ranks" for a summary of the ranks and "test" for test results. For the to_SPSS method, only one option is allowed (the default is the table of test results), but the print method allows several options (the default is to print all tables).

version

a character string specifying whether the table should mimic the content and look of recent SPSS versions ("modern") or older versions (<24; "legacy"). The main difference in terms of content is that small p-values are displayed differently.

digits

for the to_SPSS method, an integer giving the number of digits after the comma to be printed in the SPSS table. For the print method, this should be an integer vector of length 2, with the first element corresponding to the number of digits in table with the summary of the ranks, and the second element corresponding to the number of digits in the table for the test.

...

additional arguments to be passed down to format_SPSS.

Details

The print method first calls the to_SPSS method followed by to_latex. Further customization can be done by calling those two functions separately, and modifying the object returned by to_SPSS.

Value

An object of class "wilcoxon_test_SPSS" with the following components:

statistics

a data frame containing the relevant information on the ranks.

test

a list containing the results of the Wilcoxon signed rank test (only paired-sample test).

variables

a character vector containing the name(s) of the relevant numeric variable(s).

n

an integer giving the number of observations (only paired-sample test).

u

numeric; the Mann-Whitney U test statistic (only independent-samples test).

w

numeric; the Wilcoxon rank sum test statistic (only independent-samples test).

asymptotic

a list containing the results of the Wilcoxon rank sum test using the normal approximation (only independent-samples test).

exact

if requested, the corresponding p-value of the exact Wilcoxon rank sum test test (only independent-samples test).

group

a character string containing the name of the grouping variable (only independent-samples test).

type

a character string giving the type of Wilcoxon test performed "paired" or "independent").

The to_SPSS method returns an object of class "SPSS_table" which contains all relevant information in the required format to produce the LaTeX table. See to_latex for possible components and how to further customize the LaTeX table based on the returned object.

The print method produces a LaTeX table that mimics the look of SPSS output.

Note

The Wilcoxon rank sum test also reports the value of the equivalent Mann-Whitney U test statistic.

LaTeX tables that mimic recent versions of SPSS (version = "modern") may require several LaTeX compilations to be displayed correctly.

Author(s)

Andreas Alfons

Examples

## paired sample

# load data
data("Exams")

# test whether grades differ between the
# regular exam and the resit
wilcoxon_test(Exams, c("Regular", "Resit"))


## independent samples

# load data
data("Eredivisie")

# test whether market values differ between Dutch and foreign
# players
wilcoxon_test(Eredivisie, "MarketValue", group = "Foreign")