R: Test for Equal Means in a One-Way Layout

oneway.test {ctest}

R Documentation

Test for Equal Means in a One-Way Layout

Description

Test whether two or more samples from normal distributions have the same means. The variances are not necessarily assumed to be equal.

Usage

oneway.test(formula, data, subset, na.action, var.equal = FALSE)

Arguments

formula

a formula of the form lhs ~ rhs where lhs gives the sample values and rhs the corresponding groups.

data

an optional data frame containing the variables in the model 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").

var.equal

a logical variable indicating whether to treat the variances in the samples as equal. If TRUE, then a simple F test for the equality of means in a one-way analysis of variance is preformed. If FALSE, an approximate method of Welch (1951) is used, which generalizes the commonly known 2-sample Welch test to the case of arbitrarily many samples.

Value

A list with class "htest" containing the following components:

statistic

the value of the test statistic.

parameter

the degrees of freedom of the exact or approximate F distribution of the test statistic.

p.value

the p-value of the test.

method

a character string indicating the test performed.

data.name

a character string giving the names of the data.

References

B. L. Welch (1951), On the comparison of several mean values: an alternative approach. Biometrika, 38, 330–336.

Examples

data(sleep)
## Not assuming equal variances
oneway.test(extra ~ group, data = sleep)
## Assuming equal variances
oneway.test(extra ~ group, data = sleep, var.equal = TRUE)
## which gives the same result as
anova(lm(extra ~ group, data = sleep))