modlevenes.test {asbio} | R Documentation |
Conducts the modifies Levene's test for homoscedastic populations.
modlevenes.test(x, groups)
x |
Vector of residuals from a linear model. |
groups |
Vector of factor levels. |
The modified Levene's test is a test for homoscedsticity 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 < α, we reject the null and conclude that the population error variances are not equal.
An ANOVA table is returned with the modified Levene's test results.
Ken Aho
Kutner, M. H., Nachtsheim, C. J., Neter, J., and W. Li. (2005) Applied linear statistical models, 5th edition. McGraw-Hill, Boston.
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) modlevenes.test(residuals(lm1),trt)