modlevene.test {asbio}

Modified Levene's test

Description

Conducts the modified Levene's test for homoscedastic populations.

Usage

modlevene.test(y, x)

Arguments

y

Vector of quantitative ressponses, e.g., residuals from a linear model.

x

Vector of factor levels.

Details

The modified Levene's test is a test for homoscedasticity that (unlike the classic F-test) is robust to violations of normality (Conover et al. 1981). In a Modified Levene's test we calculate d_{ij}=|e_{ij} - \tilde{e}_{i}| where \tilde{e}_i is the ith factor level residual median. We then run an ANOVA on the d_{ij}'s. If the p-value is < \alpha, we reject the null and conclude that the population error variances are not equal.

Value

An ANOVA table is returned with the modified Levene's test results.

Author(s)

Ken Aho

References

Kutner, M. H., Nachtsheim, C. J., Neter, J., and W. Li. (2005) Applied Linear Statistical Models, 5th edition. McGraw-Hill, Boston.

See Also

fligner.test

Examples

eggs<-c(11,17,16,14,15,12,10,15,19,11,23,20,18,17,27,33,22,26,28)
trt<-as.factor(c(1,1,1,1,1,2,2,2,2,2,3,3,3,3,4,4,4,4,4))
lm1<-lm(eggs~trt)
modlevene.test(residuals(lm1),trt)