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Re: st: Using ml with hard-coded variables.


From   "Nick B." <3kdo02s@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Using ml with hard-coded variables.
Date   Fri, 18 May 2007 02:25:56 -0400

Thanks, that works!

However, when I try to step it up to the non-linear version, I
repeatedly get "could not calculate numerical derivatives / flat or
discontinuous region encountered".

That is, when I change the key line (being sure to add the extra
parameters to the appropriate other lines) from:

quietly gen double `p'=normal(`theta1'*mpg +`theta2'*price +`beta1')

to

quietly gen double `p'=normal(`theta1'*mpg^`theta3'
+`theta2'*price^`theta3' +`beta1')

it does converge to produce estimates, but

quietly gen double `p'=normal((`theta1'*mpg)^`theta3'
+(`theta2'*price)^`theta3' +`beta1')

produces the "could not calculate..." error, as does:

quietly gen double `p'=normal((`theta1'*mpg^`theta3'
+`theta2'*price^`theta3')^`theta4' +`beta1')

Am I still making basic mistakes, or is this due to issues with the
optimization algorithm?  How do I get around this if I want to
estimate a model like the last one above?

Thanks again for your help.

On 5/18/07, Partha Deb <partha.deb@hunter.cuny.edu> wrote:
Oops, the curse of the too-quick response.

This should do it.

sysuse auto

capture program drop probit1
program define probit1
     args lnf theta1 beta1
     tempvar p
     quietly gen double `p'=norm(`theta1'*mpg + `beta1')
     quietly replace `lnf' = $ML_y1*ln(`p')+(1-$ML_y1)*ln(1-`p')
end

ml model lf probit1 (foreign=) /beta1
ml max




Nick B. wrote:
> Well, the suggested fix does produce an estimate, but 1) it only
> estimates b0 (the constant) rather than both b0 and mpg's b1, and 2)
> the b0 estimate doesn't match either beta from "probit foreign mpg".
>
> So I'm still stuck on how to make the basic hard-coded ml probit work...
>
> On 5/17/07, Partha Deb <partha.deb@hunter.cuny.edu> wrote:
>> Nick,
>>
>> At least for your simplified model, changing
>>
>> ml model lf probit1 (foreign=) / beta1
>>
>> to
>>
>> ml model lf probit1 (foreign=)
>>
>> fixes the problem.  There's only one parameter in your model, and it is
>> implied by the constant specified in foreign= .  You might have subtle
>> singularities in your more "complicated" model.
>>
>> Hope this helps.
>>
>> Partha
>>
>>
>> Nick B. wrote:
>> > I'm a beginner here, trying to use Stata's ml to estimate a
>> > probit-like equation with non-linear terms.  However, whenever I try
>> > using hard-coded independent variables, I get the following error:
>> >
>> > initial:       log likelihood = -51.292891
>> > alternative:   log likelihood =  -1877.398
>> > rescale:       log likelihood = -48.015768
>> > rescale eq:    log likelihood = -48.015768
>> > could not calculate numerical derivatives
>> > flat or discontinuous region encountered
>> >
>> > Even when I simplify my system, I still get these errors, so I must be
>> > doing something wrong.
>> >
>> > For instance, using the built-in auto data, I try
>> > -----
>> > sysuse auto
>> > capture program drop probit1
>> >
>> > program define probit1
>> >     args lnf theta1
>> >     tempvar p
>> >     quietly gen double `p'=norm(`theta1'*mpg)
>> >     quietly replace `lnf' = $ML_y1*ln(`p')+(1-$ML_y1)*ln(1-`p')
>> > end
>> >
>> > ml model lf probit1 (foreign=) /beta1
>> > ml max
>> > ------
>> > This produces the error, even though it is almost identical to the
>> > textbook example for using ml for probit.  Specifically, replacing the
>> > relevant lines with the non-hard-coded version
>> >
>> > quietly gen double `p'=norm(`theta1')
>> > ml model lf probit1 (foreign=mpg)
>> >
>> > works fine.  So I assume I am doing something basic wrong in switching
>> > to hard-coded variables.
>> >
>> >
>> > The reason I need the hard-coded variables is that what I'm really
>> > trying to estimate is closer to
>> > ----
>> > capture program drop probit2
>> >
>> > program define probit2
>> >     args lnf theta1 theta2 theta3 theta4
>> >     tempvar p
>> >     quietly gen double
>> > `p'=norm(((`theta1'*mpg)^`theta3'+(`theta2'*length)^`theta3')^`theta4')
>> >     quietly replace `lnf' = $ML_y1*ln(`p')+(1-$ML_y1)*ln(1-`p')
>> > end
>> >
>> > ml model lf probit2 (foreign=) /beta1 /beta2 /gamma1 /gamma2
>> >
>> > ml max
>> > ----
>> > Which of course produces the same error.
>> >
>> > Any help would be greatly appreciated.
>> > *
>> > *   For searches and help try:
>> > *   http://www.stata.com/support/faqs/res/findit.html
>> > *   http://www.stata.com/support/statalist/faq
>> > *   http://www.ats.ucla.edu/stat/stata/
>>
>> --
>> Partha Deb
>> Department of Economics
>> Hunter College
>> ph:  (212) 772-5435
>> fax: (212) 772-5398
>> http://urban.hunter.cuny.edu/~deb/
>>
>> Emancipate yourselves from mental slavery
>> None but ourselves can free our minds.
>>         - Bob Marley
>>
>> *
>> *   For searches and help try:
>> *   http://www.stata.com/support/faqs/res/findit.html
>> *   http://www.stata.com/support/statalist/faq
>> *   http://www.ats.ucla.edu/stat/stata/
>>
> *
> *   For searches and help try:
> *   http://www.stata.com/support/faqs/res/findit.html
> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/

--
Partha Deb
Department of Economics
Hunter College
ph:  (212) 772-5435
fax: (212) 772-5398
http://urban.hunter.cuny.edu/~deb/

Emancipate yourselves from mental slavery
None but ourselves can free our minds.
        - Bob Marley

*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/

*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/



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