var_tests: Variance Equality Tests

Description

Displayed sample sizes and SDs and performs Brown-Forsythe and Fligner-Killeen variance equality tests (tests of homogeneity of variances) per group combinations. This is primarily a subfunction of anova_neat, but here it is available separately for other potential purposes.

Usage

var_tests(xvar, group_by, dat = NULL, hush = FALSE, sep = ", ")

Value

Prints test results.

Arguments

xvar: Either a numeric vector (numbers of any given variable), or, if dat is given, a column name specifying the variable in the given data frame.
group_by: Either a vector of factors with which to group the xvar values, or, if dat is given, one or more column names specifying the columns in the given data frame.
dat: Either NULL or a data frame from which the respective column names should be selected for xvar and group.
hush: Logical. If TRUE, prevents printing any details to console.
sep: String (underscore "_" by default) for separating group names.

References

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

Conover W. J., Johnson M. E., & Johnson M. M. (1981). A comparative study of tests for homogeneity of variances, with applications to the outer continental shelf bidding data. Technometrics, 23, 351–361.

Fligner, M. A. & Killeen, T. J. (1976). Distribution-free two-sample tests for scale. ‘Journal of the American Statistical Association. 71(353), 210-213.

Fox, J. & Weisberg, S. (2019) An R Companion to Applied Regression, Third Edition, Sage.

Levene, H. (1960). Robust tests for equality of variances. In I. Olkin, H. Hotelling, et al. (eds.). Contributions to Probability and Statistics: Essays in Honor of Harold Hotelling. Stanford University Press. pp. 278–292.

Examples

Run this code


data("ToothGrowth") # load base R example dataset

# the statistics of the four functions below should match
var_tests(ToothGrowth$len, ToothGrowth$supp)
var_tests('len', 'supp', ToothGrowth)
car::leveneTest(len ~ supp, data = ToothGrowth)
stats::fligner.test(len ~ supp, ToothGrowth)

# again the results below should match each other
var_tests(ToothGrowth$len,
          interaction(ToothGrowth$supp, ToothGrowth$dose))
var_tests('len', c('supp', 'dose'), ToothGrowth)
car::leveneTest(len ~ supp * as.factor(dose), data = ToothGrowth)
stats::fligner.test(len ~ interaction(supp, dose), ToothGrowth)