Using absorb with regress: undocumented, yet usable?

[Benoit Schmutz]

Dear all, I used the absorb option with regress (on Stata 17 -- apparently it is no longer possible on later versions) and Stata returns an output. I would like to understand what Stata does that differs from areg or reghdfe. For example, Stata keeps all the data, including singletons. Is this undocumented option a bug? Is it demeaning in some way? thank you

[Carlo Lazzaro]

Benoit:
welcome to this forum.
It does the very same job of -areg- (that, in turns, gives back more information on -fe-):

. sysuse auto.dta
(1978 automobile data)

. regress price mpg, abs(foreign)

Linear regression, absorbing indicators         Number of obs     =         74
                                                F(1, 71)          =      27.91
                                                Prob > F          =     0.0000
                                                R-squared         =     0.2838
                                                Adj R-squared     =     0.2637
                                                Root MSE          =     2530.9

------------------------------------------------------------------------------
       price | Coefficient  Std. err.      t    P>|t|     [95% conf. interval]
-------------+----------------------------------------------------------------
         mpg |  -294.1955   55.69172    -5.28   0.000    -405.2417   -183.1494
       _cons |   12430.83    1222.03    10.17   0.000     9994.169    14867.48
------------------------------------------------------------------------------


. areg price mpg, abs(foreign)

Linear regression, absorbing indicators          Number of obs     =        74
Absorbed variable: foreign                       No. of categories =         2
                                                 F(1, 71)          =     27.91
                                                 Prob > F          =    0.0000
                                                 R-squared         =    0.2838
                                                 Adj R-squared     =    0.2637
                                                 Root MSE          = 2530.9456

------------------------------------------------------------------------------
       price | Coefficient  Std. err.      t    P>|t|     [95% conf. interval]
-------------+----------------------------------------------------------------
         mpg |  -294.1955   55.69172    -5.28   0.000    -405.2417   -183.1494
       _cons |   12430.83    1222.03    10.17   0.000     9994.169    14867.48
------------------------------------------------------------------------------
F test of absorbed indicators: F(1, 71) = 6.371               Prob > F = 0.014

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