use http://www.stata-press.com/data/r8/womenwk, clear
generate d = 1
replace d = 0 if wage == .
heckman wage educ age, select (married children educ age) twostep
heckman wage educ age, select (married children educ age) nolog
*heckman wage educ age, select (d = married children educ age) twostep nolog
*probit d married children educ age, nolog
predict cndwage, ycond
summarize wage cndwage if wage < .
predict expwage, yexpected
gen wage0 = wage
replace wage0 = 0 if wage >= .
summarize wage0 expwage
这其中的三个空格加点是什么意思啊?都是空缺值或0的意思吗?