# Example: Computer-assisted learning
# A study of computer-assisted learning in 12 students investigated the
# relationship between
# Cost: cost of computer time (in cents)
# Responses: the total number of responses in completing a lesson

dados<-scan("http://www.stat.ufl.edu/~winner/sta4210/data/CH11PR06.txt", list(cost=0, responses=0))

attach(dados)

plot(responses,cost, pch=16)

fit.model<-lm(cost~responses)

library(MASS)
plot(fitted(fit.model),stdres(fit.model), pch=16)

plot(responses,stdres(fit.model), pch=16)

fit.model<-lm(cost~responses, weight=1/(responses^2))

plot(fitted(fit.model),stdres(fit.model),pch=16)