bartlett.test {stats}  R Documentation 
Performs Bartlett's test of the null that the variances in each of the groups (samples) are the same.
bartlett.test(x, ...) ## Default S3 method: bartlett.test(x, g, ...) ## S3 method for class 'formula' bartlett.test(formula, data, subset, na.action, ...)
x 
a numeric vector of data values, or a list of numeric data
vectors representing the respective samples, or fitted linear model
objects (inheriting from class 
g 
a vector or factor object giving the group for the
corresponding elements of 
formula 
a formula of the form 
data 
an optional matrix or data frame (or similar: see

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 
... 
further arguments to be passed to or from methods. 
If x
is a list, its elements are taken as the samples or fitted
linear models to be compared for homogeneity of variances. In this
case, the elements must either all be numeric data vectors or fitted
linear model objects, g
is ignored, and one can simply use
bartlett.test(x)
to perform the test. If the samples are not
yet contained in a list, use bartlett.test(list(x, ...))
.
Otherwise, x
must be a numeric data vector, and g
must
be a vector or factor object of the same length as x
giving the
group for the corresponding elements of x
.
A list of class "htest"
containing the following components:
statistic 
Bartlett's Ksquared test statistic. 
parameter 
the degrees of freedom of the approximate chisquared distribution of the test statistic. 
p.value 
the pvalue of the test. 
method 
the character string

data.name 
a character string giving the names of the data. 
Bartlett, M. S. (1937). Properties of sufficiency and statistical tests. Proceedings of the Royal Society of London Series A 160, 268–282. doi: 10.1098/rspa.1937.0109.
var.test
for the special case of comparing variances in
two samples from normal distributions;
fligner.test
for a rankbased (nonparametric)
ksample test for homogeneity of variances;
ansari.test
and mood.test
for two rank
based twosample tests for difference in scale.
require(graphics) plot(count ~ spray, data = InsectSprays) bartlett.test(InsectSprays$count, InsectSprays$spray) bartlett.test(count ~ spray, data = InsectSprays)