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Re: st: the xi: command --- an uglier hack


From   Mark Schaffer <M.E.Schaffer@hw.ac.uk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: the xi: command --- an uglier hack
Date   Fri, 05 Jul 2002 16:16:40 +0100 (BST)

On Fri, 05 Jul 2002 10:44:50 -0400 (EDT), Michael Ash 
<mash@econs.umass.edu> wrote:

> > I am using the xi: command to construct dummy variables
> > from categorical variables. My problem is that I would
> > like the xi-command to not remove one of the dummies...
> 
> > I just want to add that I've run into this problem too,
> > and I too consider it a (rare) limitation.  More than
> > once I've wanted to run a regression with explicit
> > dummies for each category in combination with the
> > -nocons- option, and without an ugly hack not worth
> > repeating here (Uli's suggestion is *much* nicer) it
> > isn't possible with -xi-'s dummies.
> 
> I agree that the "tab, gen()" solution is much nicer, but
> here is the "ugly" hack.  The following command "xa" works
> exactly like "xi" except it does not omit any category.  I
> believe that it does not interfere with the normal operation
> of xi. Comments welcome.
> 
> -Michael Ash
> 
> ------xa.ado-----CUT BELOW THIS LINE-------
> *! version 1.1.0   10/19/94
> program define xa
> 	version 4.0

<snip>

Actually, the ugly hack I had in mind was uglier but shorter.

Say the categorical variable is called -industry-, which in the 
dataset takes either positive integer values or is missing.

I would create an additional observation in the data that has a 
missing value for every variable except -industry-.  The variable
-industry- takes the value 0 for this observation alone.

I would then use the command

char industry[omit] 0

so that this observation is the benchmark for the dummies created by
-xi-.

I never found any need to drop this phantom observation, though of 
course this could be done after the -xi- command creates the dummies.

Well, I *did* say it was an ugly hack.  It works, though.

--Mark

Prof. Mark Schaffer
Director, CERT
Department of Economics, School of Management
Heriot-Watt University, Edinburgh EH14 4AS
tel +44-131-451-3494 / fax +44-131-451-3008
email: m.e.schaffer@hw.ac.uk
web: http://www.som.hw.ac.uk/ecomes
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