看heckman的帮助,
看看人家的if是写在哪里?
Title
[R] heckman -- Heckman selection model
Syntax
Basic syntax
heckman depvar [indepvars], select(varlist_s) [twostep]
or
heckman depvar [indepvars], select(depvar_s = varlist_s) [twostep]
Full syntax for maximum likelihood estimates only
heckman depvar [indepvars] [if] [in] [weight], select([depvar_s =] varlist_s [, noconstant offset(
varname_o)]) [heckman_ml_options]
Full syntax for Heckman's two-step consistent estimates only
heckman depvar [indepvars] [if] [in], twostep select([depvar_s =] varlist_s [, noconstant])
[heckman_ts_options]