LeveneTest: Levene's Test for Homogeneity of Variance

Description

Computes Levene's test for homogeneity of variance across groups.

Usage

LeveneTest(y, ...)

## S3 method for class 'formula':
LeveneTest(y, data, ...)
## S3 method for class 'lm':
LeveneTest(y, ...)
## S3 method for class 'default':
LeveneTest(y, group, center=median, ...)

Arguments

response variable for the default method, or a lm or formula object. If y is a linear-model object or a formula, the variables on the right-hand-side of the model must all be factors and must be completely

group

factor defining groups.

center

The name of a function to compute the center of each group; mean gives the original Levene's test; the default, median, provides a more robust test (Brown-Forsythe-Test).

data

a data frame for evaluating the formula.

...

arguments to be passed down, e.g., data for the formula and lm methods; can also be used to pass arguments to the function given by center (e.g., center=mean and trim=0.1

`Value`

returns an object meant to be printed showing the results of the test.

`References`

Fox, J. (2008)
  Applied Regression Analysis and Generalized Linear Models,
  Second Edition. Sage.  
  
  Fox, J. and Weisberg, S. (2011) 
  An R Companion to Applied Regression, Second Edition, Sage.

`See Also`

fligner.test for a rank-based (nonparametric)
  $k$-sample test for homogeneity of variances;
  mood.test for another rank-based two-sample test for a
  difference in scale parameters;
  var.test and bartlett.test for parametric
  tests for the homogeneity in variance.

  ansari_test in package {coin}
  for exact and approximate conditional p-values for the
  Ansari-Bradley test, as well as different methods for handling ties.

`Examples`

Run this code## example from ansari.test:
## Hollander & Wolfe (1973, p. 86f):
## 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)

LeveneTest( c(ramsay, jung.parekh), 
  factor(c(rep("ramsay",length(ramsay)), rep("jung.parekh",length(jung.parekh)))))

LeveneTest( c(rnorm(10), rnorm(10, 0, 2)), factor(rep(c("A","B"),each=10)) )

# original example from package car

with(Moore, LeveneTest(conformity, fcategory))
with(Moore, LeveneTest(conformity, interaction(fcategory, partner.status)))

LeveneTest(conformity ~ fcategory * partner.status, data = Moore)
LeveneTest(conformity ~ fcategory * partner.status, data = Moore, center = mean)
LeveneTest(conformity ~ fcategory * partner.status, data = Moore, center = mean, trim = 0.1)

LeveneTest(lm(conformity ~ fcategory*partner.status, data = Moore))
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