Example 21.2 Tobit Analysis
The following statements show a subset of the Mroz (1987) data set. In these data, Hours is the number of hours the wife worked outside the household in a given year, Yrs_Ed is the years of education, and Yrs_Exp is the years of work experience. A Tobit model will be fit to the hours worked with years of education and experience as covariates.
By the nature of the data it is clear that there are a number of women who committed some positive number of hours to outside work (
is observed). There are also a number of women who did not work at all (
is observed). This gives us the following model:
where
. The set of explanatory variables is denoted by
.
title1 'Estimating a Tobit model';
data subset;
input Hours Yrs_Ed Yrs_Exp @@;
if Hours eq 0 then Lower=.;
else Lower=Hours;
datalines;
0 8 9 0 8 12 0 9 10 0 10 15 0 11 4 0 11 6
1000 12 1 1960 12 29 0 13 3 2100 13 36
3686 14 11 1920 14 38 0 15 14 1728 16 3
1568 16 19 1316 17 7 0 17 15
;
/*-- Tobit Model --*/
proc qlim data=subset;
model hours = yrs_ed yrs_exp;
endogenous hours ~ censored(lb=0);
run;
The output of the QLIM procedure is shown in Output 21.2.1.
Output 21.2.1
Tobit Analysis Results
1 |
Hours |
17 |
-74.93700 |
1.18953E-6 |
23 |
Quasi-Newton |
157.87400 |
161.20685 |
1 |
-5598.295129 |
27.692220 |
-202.16 |
<.0001 |
1 |
373.123254 |
53.988877 |
6.91 |
<.0001 |
1 |
63.336247 |
36.551299 |
1.73 |
0.0831 |
1 |
1582.859635 |
390.076480 |
4.06 |
<.0001 |