library(MASS)
data(crabs)

help(model.matrix)
X <- model.matrix(CL~FL+sex,data=crabs)
X

model1 <- lm(CL~FL+sex,data=crabs)
summary(model1)

crabs$sex <- relevel(crabs$sex,"M")
model2 <- lm(CL~FL+sex,data=crabs)
summary(model2)

model3 <- lm(CL~FL+sex-1,data=crabs)
summary(model3)

contrasts(crabs$sex)=contr.sum(levels(crabs$sex))
model4 <- lm(CL~FL+sex,data=crabs)
summary(model4)