On Mon, 10 Dec 2007, Allin Cottrell wrote:
On Thu, 6 Dec 2007, Riccardo (Jack) Lucchetti wrote:
> I was just unaware of the fact that if the constant is in the
> regressor list, it gets automatically added to the instrument
> list if not already present...
>
> I think I can see Allin's reasoning here "how can the constant
> be endogenous?", but I wonder if this is maybe going a little
> too far in protecting the user from his own stupidity. I can't
> imagine right now a practical situation when the constant needs
> to be among the regressors but not among the instruments, but
> hey, it's a free world.
I don't feel strongly about this, but my first reaction is that
it's a low-cost convenience to the user, to add the constant to
the list of instruments if it's among the regressors (and not
already specified as an instrument). If we're to remove this,
we'd have to check that it doesn't break TSLS in the GUI.
Good point.
I tend to think that if a user really wants to treat the constant
as endogenous, s/he can go ahead and do the two stages of TSLS
manually.
I don't have a very strong opinion on this either, but I think that in a
script it looks weird if you have more regressors than instruments. At
least _my_ first reaction was when I saw Sven's example to think that tsls
was broken.
Sure adding the constant makes sense in those cases, but so does in
various other contexts. For example, when you run a regression in Stata,
the constant is always added automagically unless you specify the
"noconstant" option. We do the same for ARIMAs, so one may conceivably
consider adding the constant by default in all estimation methods,
including ols, for consistency.
The fact remains, that if a user wanted, maybe just to see what happens,
to run a weird tsls model with a constant among the regressors but not
among the instruments, s/he has to do it by hand (which is, I agree, only
a minor inconvenience).
If we decide to keep it in, fine by me, but I'd like to have some warning
message printed out, as per my recent CVS commit (the exact wording could
be softened).
Riccardo (Jack) Lucchetti
Dipartimento di Economia
Università Politecnica delle Marche
r.lucchetti(a)univpm.it
http://www.econ.univpm.it/lucchetti