*
* Example 9.10 on page 314-315
*
open data infmrt.raw
data(format=free,org=columns) 1 102 year infmort afdcprt popul pcinc physic afdcper d90 $
  lpcinc lphysic dc lpopul
*
linreg(smpl=d90) infmort
# constant lpcinc lphysic lpopul
linreg(smpl=d90.and..not.dc) infmort
# constant lpcinc lphysic lpopul