*
*  Example 4.4
*  J. Wooldridge, Econometrics of Cross Section and Panel Data
*
*  Note: The sample data files include all required data transformations.
*        However, in many cases we show how these data transformations would
*        be created in RATS from the basic data, as this is something you will
*        need to know how to do in practice.
*
cal(panelobs=3) 1987
all 157//3
open data jtrain1.raw
data(format=free,org=columns) / year fcode employ sales	avgsal scrap rework tothrs $
 union grant d89 d88 totrain hrsemp
*
* Set a dummy which is 1 only if the year is 1988 and there is a value for scrap.
*
set scrap1988 = year==1988.and.%valid(scrap)
set lscrap = log(scrap)
*
* Run the two regressions, restricting the sample only to the entries where
* "scrap1988" is 1.
*
linreg(smpl=scrap1988) lscrap
# constant grant
*
linreg(smpl=scrap1988) lscrap
# constant grant lscrap{1}