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Re: st: mysureg with constraints


From   jpitblado@stata.com (Jeff Pitblado, StataCorp LP)
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: mysureg with constraints
Date   Tue, 20 Sep 2005 14:17:38 -0500

Federica Maiorano <federica_maiorano@yahoo.it> seems to be having some trouble
with constraints:

> I am estimating a translog cost function + 1 input
> share cost equation using mysureg.
> 
> When I attempt to estimate a constrained model, then I
> get an error message from mysureg. As an example, I
> copy below what happens with one constraint only. 
> 
> Please note that there do not seem to be any problems
> when the same commands are performed using sureg. 
> 
> I am obviously doing something wrong, but I cannot
> work out what it is. Any suggestions?
> 
> -----------------------------------------------
> constraint define 1 [sl]_cons = [lnvc]lnpl
> 
> mysureg (lnvc lnk lnpl lnpm lnaccess lncalls time k2
> pl2 pm2 acc2 call2 t2 kpl kpm kacc kcalls kt plpm
> placc plcalls plt pmacc pmcalls pmt acccalls acct
> callt) (sl lnpl lnpm lnaccess lncalls lnk time),
> constraint(1) cluster(idcomp)
> 
> Fitting constant-only model:
> Constraints invalid:
> [lnpl] not found
> 
> -----------------------------------------------

Aside from being an example estimation command in the Stata Press book 
"Maximum Likelihood Estimation with Stata, 2nd Edition", there does not seem
to be a publicly available user-written estimation command called -mysureg-.
So I'll assume that Federica is using -mysureg- introduced on pg. 238 (the
ado-file for -mysureg- is given on pg.  286).

	Federica discovered a bug in -ml model- that was fixed for Stata 9 in
	the ado-file update on 15sep2005.

If Federica is using Stata 9, she should use -update- to get Stata up-to-date,
and the problem will go away.

For Stata 8, Federica can change -mysureg- to parse the -constraints()- option
as a "passthru", and add `constraints' back to the `mlopts' macro after
-mysureg- fits the constant-only model.  Here are the lines that will be
affected (look for !!):

***** BEGIN: mysureg.ado
program mysureg, sortpreserve
	...
	syntax anything(id="equations" equalok) ///
		[if] [in] [fweight pweight] [,  ///
		noLOg				/// -ml model- options
		Robust CLuster(varname)		///
		noLRTEST svy noSVYadjust	///
		Level(integer `c(level)')       /// -Replay- options
		corr				///
		CONSTraints(passthru)		/// !! NEW LINE
		*				/// -mlopts/svyopts- options
	]
	...
	mlopts mlopts, `options'
	local cns `constraints'			// !! LINE MODIFIED
	...

	`qui' di as txt _n "Fitting constant-only model:"
	ml model d2 mysuregc_d2 `eqns0'		///
		...
		nocnsnotes missing maximize
	local initopt continue search(off)
	if "`lrtest'" == "" {
		local lf0 lf0(`k_eq' `e(ll)')
	}
	local mlopts `"`mlopts' `constraints'"'	// !! NEW LINE
	...
end
...
***** END:   mysureg.ado

So, you ask: "What's going on?  I want details."

As originally developed, -mysureg- passes constraints to each call of
-ml model-, even when fitting the constant-only model.  Under version 8.1 (and
8.2 for Stata 8), -ml model- does not use -makecns- to tolerantly construct the
constraint matrix; it stops with an error message instead of just noting
when there was a problem with a constraint then move on.  Thus -ml model-
exits with an error if any of the constraints involve predictors that are not
present in the constant-only model fit.  As mentioned above, this bug was fixed
in Stata 9.

--Jeff
jpitblado@stata.com
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