#######
# DID #
#######

install.packages("wooldridge")
data(kielmc, package='wooldridge')
head(kielmc)
coef(lm(rprice~nearinc, data=kielmc, subset=(year==1978)))
coef(lm(rprice~nearinc, data=kielmc, subset=(year==1981)))
did = -30688.27-(-18824.37)
did

interaction = kielmc$nearinc*kielmc$y81
summary(lm(rprice~nearinc+y81+interaction, data=kielmc))$coefficients

summary(lm(rprice~nearinc+y81+interaction+area+age+baths, data=kielmc))$coefficients