library(sampleSelection)
#estimar Heckman versao 2 estagios
res<-selection(inlf~educ+exper+I(exper^2)+nwifeinc+age+kidslt6+kidsge6,log(wage)~educ+exper+I(exper^2),data=MROZ, method="2step")
summary(res)
#estimar Heckman versao ML
res1<-selection(inlf~educ+exper+I(exper^2)+nwifeinc+age+kidslt6+kidsge6,log(wage)~educ+exper+I(exper^2),data=MROZ, method="ml")
summary(res1)