Dear everyone:
I would like to know, with STATA commands, whether Tobit TYPE-I and
Type-II (i.e., heckman selection model) will provide the same estimation
results if in the Type-II the regression equation has the same set of
variables with the selection equation. For example, as described below,
I compared the means of predicted wages with "tobit" command for TYPE I
and "heckman" command for Type II. However, they provide the different
results.
I would appreciate if anyone can tell me: whether my understanding for
the relation between Tobit TYPE I and TYPE II is wrong, or whether the
following STATA commands are wrong for my purpose. If the latter is the
case, could you also tell me how to do?
.use http://stata-press.com/data/r8/womenwk
(a) For Type I
.gen wage1=0
.replace wage1=wage if wage~=.
.tobit wage1 educ age, ll(0)
. predict yhatwage1
. sum yhatwage1
Variable | Obs Mean Std. Dev. Min Max
-------------+--------------------------------------------------------
yhatwage1 | 2000 12.70266 6.876089 -.4998228 34.74677
(b) For Type II
.heckman wage educ age, select (educ age)
. predict yhatwage
. sum wage yhatwage
Variable | Obs Mean Std. Dev. Min Max
-------------+--------------------------------------------------------
wage | 1343 23.69217 6.305374 5.88497 45.80979
yhatwage | 2000 20.93415 3.953325 14.20508 33.01097
Thank you very much for any help.
Takako Yuki
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