misty (version 0.6.7)

test.levene: Levene's Test for Homogeneity of Variance

Description

This function performs Levene's test for homogeneity of variance across two or more independent groups.

Usage

test.levene(formula, data, method = c("median", "mean"), conf.level = 0.95,
            hypo = TRUE, descript = TRUE, plot = FALSE, violin.alpha = 0.3,
            violin.trim = FALSE, box = TRUE, box.alpha = 0.2, box.width = 0.2,
            jitter = TRUE, jitter.size = 1.25, jitter.width = 0.05,
            jitter.height = 0, jitter.alpha = 0.2, gray = FALSE,
            start = 0.9, end = 0.4, color = NULL, xlab = NULL, ylab = NULL,
            ylim = NULL, breaks = ggplot2::waiver(), title = "",
            subtitle = "", digits = 2, p.digits = 3, as.na = NULL,
            write = NULL, append = TRUE, check = TRUE, output = TRUE)

Value

Returns an object of class misty.object, which is a list with following entries:

call

function call

type

type of analysis

formula

formula of the current analysis

data

data frame specified in data

plot

ggplot2 object for plotting the results

args

specification of function arguments

result

list with result tables, i.e., descript for descriptive statistics and test for the ANOVA table

Arguments

formula

a formula of the form y ~ group where y is a numeric variable giving the data values and group a numeric variable, character variable or factor with two or more than two values or factor levels giving the corresponding groups.

data

a matrix or data frame containing the variables in the formula formula.

method

a character string specifying the method to compute the center of each group, i.e. method = "median" (default) to compute the Levene's test based on the median (aka Brown-Forsythe test) or method = "mean" to compute the Levene's test based on the arithmetic mean.

conf.level

a numeric value between 0 and 1 indicating the confidence level of the interval.

hypo

logical: if TRUE (default), null and alternative hypothesis are shown on the console.

descript

logical: if TRUE (default), descriptive statistics are shown on the console.

plot

logical: if TRUE, a plot showing violin plots with boxplots is drawn.

violin.alpha

a numeric value indicating the opacity of the violins.

violin.trim

logical: if TRUE, the tails of the violins to the range of the data is trimmed.

box

logical: if TRUE (default), boxplots are drawn.

box.alpha

a numeric value indicating the opacity of the boxplots.

box.width

a numeric value indicating the width of the boxplots.

jitter

logical: if TRUE (default), jittered data points are drawn.

jitter.size

a numeric value indicating the size aesthetic for the jittered data points.

jitter.width

a numeric value indicating the amount of horizontal jitter.

jitter.height

a numeric value indicating the amount of vertical jitter.

jitter.alpha

a numeric value indicating the opacity of the jittered data points.

gray

logical: if TRUE, the plot is drawn in gray scale.

start

a numeric value between 0 and 1, graphical parameter to specify the gray value at the low end of the palette.

end

a numeric value between 0 and 1, graphical parameter to specify the gray value at the high end of the palette.

color

a character vector, indicating the color of the violins and the boxes. By default, default ggplot2 colors are used.

xlab

a character string specifying the labels for the x-axis.

ylab

a character string specifying the labels for the y-axis.

ylim

a numeric vector of length two specifying limits of the limits of the y-axis.

breaks

a numeric vector specifying the points at which tick-marks are drawn at the y-axis.

title

a character string specifying the text for the title for the plot.

subtitle

a character string specifying the text for the subtitle for the plot.

digits

an integer value indicating the number of decimal places to be used for displaying results.

p.digits

an integer value indicating the number of decimal places to be used for displaying the p-value.

as.na

a numeric vector indicating user-defined missing values, i.e. these values are converted to NA before conducting the analysis.

write

a character string naming a text file with file extension ".txt" (e.g., "Output.txt") for writing the output into a text file.

append

logical: if TRUE (default), output will be appended to an existing text file with extension .txt specified in write, if FALSE existing text file will be overwritten.

check

logical: if TRUE (default), argument specification is checked.

output

logical: if TRUE (default), output is shown.

Author

Takuya Yanagida

Details

Levene's test is equivalent to a one-way analysis of variance (ANOVA) with the absolute deviations of observations from the mean of each group as dependent variable (center = "mean"). Brown and Forsythe (1974) modified the Levene's test by using the absolute deviations of observations from the median (center = "median"). By default, the Levene's test uses the absolute deviations of observations from the median.

References

Brown, M. B., & Forsythe, A. B. (1974). Robust tests for the equality of variances. Journal of the American Statistical Association, 69, 364-367.

Rasch, D., Kubinger, K. D., & Yanagida, T. (2011). Statistics in psychology - Using R and SPSS. John Wiley & Sons.

See Also

aov.b, test.t, test.welch

Examples

dat <- data.frame(y = c(2, 3, 4, 5, 5, 7, 8, 4, 5, 2, 4, 3),
                  group = c(1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3))

# Example 1: Levene's test based on the median with 95% confidence interval
test.levene(y ~ group, data = dat)

# Example 2: Levene's test based on the arithmetic mean  with 95% confidence interval
test.levene(y ~ group, data = dat, method = "mean")

# Example 3: Levene's test based on the median with 99% confidence interval
test.levene(y ~ group, data = dat, conf.level = 0.99)

if (FALSE) {
# Example 4: Write results into a text file
test.levene(y ~ group, data = dat, write = "Levene.txt")

# Example 5: Levene's test based on the median with 95
# plot results
test.levene(y ~ group, data = dat, plot = TRUE)

# Load ggplot2 package
library(ggplot2)

# Save plot, ggsave() from the ggplot2 package
ggsave("Levene-test.png", dpi = 600, width = 5, height = 6)

# Levene's test based on the median with 95
# extract plot
p <- test.levene(y ~ group, data = dat, output = FALSE)$plot
p

# Example 6: Extract data
plotdat <- test.levene(y ~ group, data = dat, output = FALSE)$data

# Draw violin and boxplots in line with the default setting of test.levene()
ggplot(plotdat, aes(group, y, fill = group)) +
  geom_violin(alpha = 0.3, trim = FALSE) +
  geom_boxplot(alpha = 0.2, width = 0.2) +
  geom_jitter(alpha = 0.2, width = 0.05, size = 1.25) +
  theme_bw() + guides(fill = "none")
}