*
* Example 9.3 from page 297
*
open data wage2.raw
data(org=columns) 1 935 wage hours IQ KWW educ exper tenure age $
  married black south urban sibs brthord meduc feduc lwage
*
set educxiq = educ*iq
*
linreg lwage
# educ exper tenure married south urban black constant
linreg lwage
# educ exper tenure married south urban black iq constant
linreg lwage
# educ exper tenure married south urban black iq educxiq constant