Al, Nick,
Thanks for the advice!
Cheers,
Mark
From: "FEIVESON, ALAN H. (AL) (JSC-SD) (NASA)"
<[email protected]>
To: "'[email protected]'" <[email protected]>
Subject: st: RE: Numerical maximization
Date sent: Fri, 4 Oct 2002 10:47:22 -0500
Send reply to: [email protected]
> Mark -
>
> If you want to find a value of x that maximizes an arbitrary funcrion
> f(x), you might be able to fool -nl- into doing it, provided you can
> bound the function. Here's how. Suppose you know f(x) < f0 for all x.
> Then create a dummy data set with two observations as follows: clear
> set obs 2 gen y=1000 in 1/*(the value of f0)*/ replace y=0 in 2 /* it
> doesn't matter what y is on obs. 2 as long as it's different from f0
> */ nl max y y
>
>
> The last command calls Stata's nonlinear least square -nl- program. To
> use it you have to write your own nl-program. Here's an example where
> you are trying to maximize f(x)=x*(x+2)*exp(-0.5*x)/(x+1) for x > 0.
>
> program define nlmax
>
> if "`1'" == "?" {
> global S_1 " Z "
> global Z= 1
> exit
> }
> cap gen fh=.
> global X=exp($Z)
> replace fh=$X*(2+$X)*exp(-.5*$X)/(1+$X) in 1
> replace fh=`2' in 2
>
> replace `1' = fh
>
> end
>
> What this is doing is finding a value of Z = log(x) that makes f(x)
> come as close to 1000 as possible. Since 1000 is an upper bound on
> f(x), this will maximize f. Use of log(x) instead of x prevents the
> consideration of negative values. For example this function has a
> vertical asymptote at x = -1.
>
> Hope this helps.
>
> Al Feiveson
>
>
>
>
>
>
>
> -----Original Message-----
> From: Mark Schaffer [mailto:[email protected]]
> Sent: Wednesday, October 02, 2002 11:09 AM
> To: [email protected]
> Subject: st: Numerical maximization
>
>
> Hi everybody. A very general question for you:
>
> What's the recommended strategy if you're writing a Stata estimator
> that requires numerical maximization methods but isn't maximum
> likelihood? And if you think you might want to make the estimator
> available for general use?
>
> Stata doesn't have a maximize program as such (-maximize- seems to be
> an ML program ... but maybe it's good enough for the task?). -findit-
> told me about a couple of user-written programs called -amoeba- and
> -quasi-. I have no experience with these, nor with the tradeoffs in
> writing code that rely on other people's routines. (It's great that
> they make their code publicly available, but we can't expect them to
> refrain from changing the specs if they see fit to do so, and of
> course that could cause your own code to fail to work.)
>
> --Mark
>
> Prof. Mark E. Schaffer
> Director
> Centre for Economic Reform and Transformation
> Department of Economics
> School of Management & Languages
> Heriot-Watt University, Edinburgh EH14 4AS UK
> 44-131-451-3494 direct
> 44-131-451-3008 fax
> 44-131-451-3485 CERT administrator
> http://www.som.hw.ac.uk/cert
> *
> * 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/
Prof. Mark E. Schaffer
Director
Centre for Economic Reform and Transformation
Department of Economics
School of Management & Languages
Heriot-Watt University, Edinburgh EH14 4AS UK
44-131-451-3494 direct
44-131-451-3008 fax
44-131-451-3485 CERT administrator
http://www.som.hw.ac.uk/cert
*
* 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/
