# S&W:n kirjan kotisivut: http://wps.aw.com/aw_stock_ie_2/



datamat = t(matrix(scan("C:/HY-Data/JPLUOTO/OPETUS/EKONOMETRIAN_PERUSKURSSI/test_data.txt"),nrow=13))
datam = matrix(datamat,nc=13)
colnames(datam) = c("enrl_tot","teachers","calw_pct","meal_pct","computer","testscr","comp_stu","expn_stu","str","avginc","el_pct","read_scr","math_scr")


library(sandwich)
library(lmtest)


# Usean muuttujan regressiomalli (S&W 6)

Test_score = datam[,"testscr"]
STR = datam[,"str"]
PctEL= datam[,"el_pct"]


model = lm(Test_score~STR+PctEL)
model$newse = vcovHC(model)
coeftest(model,model$newse)
summary(model)


#Täydellinen multikollineaarisuus
model = lm(Test_score~rep(1,length(STR))+STR+PctEL)
summary(model)
model$newse = vcovHC(model)
coeftest(model,model$newse)