On Wed, 2010-07-14 at 01:37 -0700, martanair wrote:
> I am trying to use Levene's test (of package car), but I do
> not understand quite well how to use it. '?levene.test' does
> not unfortunately provide any example.
Err, yes it does, several examples in fact.
> My data are in a data
> frame and correspond to 1 factor plus response. Could
> someone please give me an example about how to use the command
>
> levene.test(y, group)
>
> Thanks in advance,
>
> marta
Here is an example where data aren't in a data.frame.
require(car)
## dummy data with know different variances
set.seed(123)
Y <- c(rnorm(20, sd = 6), rnorm(20, sd = 1))
G <- gl(2, 20, labels = paste("Group", 1:2, sep = ""))
## Y is the thing we measured on our two groups,
## G is the group indicator
## levene test of different variances
levene.test(y = Y, group = G)
## Gives:
Levene's Test for Homogeneity of Variance
Df F value Pr(>F)
group 1 21.033 4.792e-05 ***
38
---
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
If Y and G were in a data frame, then the easiest way to run it is:
## put data above in to a data frame
dat <- data.frame(Y = Y, G = G)
head(dat)
str(dat)
## ok so G is a factor within data frame
## run levene's test
with(dat, leven.test(y = Y, group = G))
?leven.test even shows you this usage!
## or if you like $
levene.test(dat$Y, dat$G)
Just replace dat Y and G with the relevant objects and components of the
data frame holding your data.
HTH
G
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