* Reference: Chapters 4, 7 and 15 of
*   Jeffrey M. Wooldridge, Introductory Econometrics: A Modern Approach,
*   South-Western College Publishing, 2000.
SAMPLE 1 471
READ (JTRAIN.shd) year fcode employ sales avgsal scrap rework tothrs &  
   union grant d89 d88 totrain hrsemp lscrap lemploy &
   lsales lrework lhrsemp lscrap_1 grant_1 clscrap cgrant &
   clemploy clsales lavgsal clavgsal cgrant_1 chrsemp clhrsemp 
SET MISSVALU=-999
SET SKIPMISS
SET NOWARNMISS

* Example 4.7, pp. 132-133
* Restrict the sample to 1987
SET NOWARNSKIP
SKIPIF (YEAR.NE.1987)
* Obtain summary statistics of the variables
* The SAMEOBS option restricts the sample to ensure that all
* observations for all variables are non-missing.
* This option is available with Version 9 - May 2002, or later versions.
STAT scrap lscrap hrsemp lsales lemploy / SAMEOBS
* OLS estimation 
OLS lscrap hrsemp lsales lemploy

* The textbook example works with a sample of 30.
* The total observations available is 43. Exercise 4.7, p. 157
* reports the estimation results with 43 observations that
* can be compared with the SHAZAM estimation results.

* Find the effect on the scrap rate for 5 additional hours of training.
TEST 5*100*hrsemp
* Again, the above result is based on a sample of 43 observations --
* the textbook example is working with 30 observations.

DELETE SKIP$

* Example 7.3
* Restrict the sample to 1988
SET NOWARNSKIP
SKIPIF (YEAR.NE.1988)
* Equation (7.7), p. 217.
OLS hrsemp grant lsales lemploy

* Section 7.6
* Equation (7.33), p. 238.
OLS lscrap grant lsales lemploy 

DELETE SKIP$
* Example 15.10
*  Consider two years 1987 and 1988.
SKIPIF (YEAR.NE.1987)
COPY scrap scrap87
COPY hrsemp hrsemp87
COPY grant grant87
DELETE SKIP$
SKIPIF (YEAR.NE.1988)
COPY scrap scrap88
COPY hrsemp hrsemp88
COPY grant grant88
DELETE SKIP$
GEN1 NOBS=471/3
SAMPLE 1 NOBS
* Calculate first differences
* Use SET NOWARN to suppress warning messages about missing
* observations with LOG.
SET NOWARN
GENR dlscrap=LOG(scrap88)-LOG(scrap87)
SET WARN
GENR dhrsemp=hrsemp88-hrsemp87
GENR dgrant=grant88-grant87
* Regression results on p. 490.
OLS dhrsemp dgrant
2SLS dlscrap dhrsemp (dgrant)
* Compare the 2SLS estimates with OLS
OLS dlscrap dhrsemp
STOP