mood.test: Mood Two-Sample Test of Scale

Description

Performs Mood's two-sample test for a difference in scale parameters.

Usage

mood.test(x, ...)
## S3 method for class 'default':
mood.test(x, y,
          alternative = c("two.sided", "less", "greater"), ...)
## S3 method for class 'formula':
mood.test(formula, data, subset, na.action, \dots)

Arguments

x, y

numeric vectors of data values.

alternative

indicates the alternative hypothesis and must be one of "two.sided" (default), "greater" or "less" all of which can be abbreviated.

formula

a formula of the form lhs ~ rhs where lhs is a numeric variable giving the data values and rhs a factor with two levels giving the corresponding groups.

data

an optional matrix or data frame (or similar: see model.frame) containing the variables in the formula formula. By default the variables are taken from environment(formula).

subset

an optional vector specifying a subset of observations to be used.

na.action

a function which indicates what should happen when the data contain NAs. Defaults to getOption("na.action").

...

further arguments to be passed to or from methods.

Value

A list with class "htest" containing the following components:
statisticthe value of the test statistic.
p.valuethe p-value of the test.
alternativea character string describing the alternative hypothesis. You can specify just the initial letter.
methodthe character string "Mood two-sample test of scale".
data.namea character string giving the names of the data.

Details

The underlying model is that the two samples are drawn from $f(x-l)$ and $f((x-l)/s)/s$, respectively, where $l$ is a common location parameter and $s$ is a scale parameter.

The null hypothesis is $s = 1$.

There are more useful tests for this problem.

In the case of ties, the formulation of Mielke (1967) is employed.

References

William J. Conover (1971), Practical nonparametric statistics. New York: John Wiley & Sons. Pages 234f.

Paul W. Mielke, Jr. (1967), Note on some squared rank tests with existing ties. Technometrics, 9/2, 312--314.

Examples

Run this code

## Same data as for the Ansari-Bradley test:
## Serum iron determination using Hyland control sera
ramsay <- c(111, 107, 100, 99, 102, 106, 109, 108, 104, 99,
            101, 96, 97, 102, 107, 113, 116, 113, 110, 98)
jung.parekh <- c(107, 108, 106, 98, 105, 103, 110, 105, 104,
            100, 96, 108, 103, 104, 114, 114, 113, 108, 106, 99)
mood.test(ramsay, jung.parekh)
## Compare this to ansari.test(ramsay, jung.parekh)

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