RinAction - Nonparametric Test of Group Difference

gistfile1.r

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# ---- Nonparametric tests of group differences ----   #
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# NOTE:
# When you have more than 2 groups, you can use ANOVA or Nonparametric approaches
# to test group difference. ANOVA assumes data are independently sampled from normal
# populations. If the parametric assumptions are not met, nonparametric method can 
# be used.


# Wilcoxon two group comparison
           
with(UScrime, by(Prob, So, median))
wilcox.test(Prob ~ So, data=UScrime)
sapply(UScrime[c("U1", "U2")], median)
with(UScrime, wilcox.test(U1, U2, paired = TRUE))
           
# Kruskal Wallis test
           
states <- as.data.frame(cbind(state.region, state.x77))
kruskal.test(Illiteracy ~ state.region, data=states)
           
# Nonparametric multiple comparisons
           
class <- state.region
var <- state.x77[, c("Illiteracy")]
mydata <- as.data.frame(cbind(class, var))
rm(class,var)
library(npmc)
summary(npmc(mydata), type = "BF")
aggregate(mydata, by = list(mydata$class), median)