library(faraway) data(coagulation) coagulation plot(coag ~ diet, coagulation, ylab="coagulation time") with(coagulation, stripchart(coag ~ diet, vertical=TRUE, method="stack",xlab="diet",ylab="coagulation time")) g <- lm(coag ~ diet, coagulation) summary(g) model.matrix(g) gi <- lm(coag ~ diet -1, coagulation) summary(gi) gnull <- lm(coag ~ 1, coagulation) anova(gnull,gi) options(contrasts=c("contr.sum","contr.poly")) gs <- lm(coag ~ diet , coagulation) summary(gs) qqnorm(residuals(g)) plot(jitter(fitted(g)),residuals(g),xlab="Fitted",ylab="Residuals") med <- with(coagulation,tapply(coag,diet,median)) ar <- with(coagulation,abs(coag -med[diet])) anova(lm(ar ~ diet,coagulation)) qt(0.975,20) c(5-2.086*1.53,5+2.086*1.53) qtukey(0.95,4,20)/sqrt(2) c(5-2.8*1.53,5+2.8*1.53) TukeyHSD(aov(coag ~ diet, coagulation)) qt(1-.05/12,20)