Re: st: nlsur aids function evaluator program

["Brian P. Poi" <bpoi@stata.com>]

Ivica Rubil wrote:
> Dear all,
> I copied the code for -nlsur- AIDS demand system from the Stata
> reference manual. But when I run it, Stata returns:
> " nlsuraids returned 101
> verify that nlsuraids1 is a function evaluator program"
> What's the problem
>
>
> Here's the code:
>
> program nlsuraids
> 	version 11
> 	
> 	syntax varlist(min=8 max=8), at(name)
> 	tokenize `varlist'
> 	args w1 w2 w3 lnp1 lnp2 lnp3 lnp4 lnexp
> 	
> 	tempname a1 a2 a3 a4
> 	scalar `a1' = `at'[1,1]
> 	scalar `a2' = `at'[1,2]
> 	scalar `a3' = `at'[1,3]
> 	scalar `a4' = 1 - `a1' - `a2' - `a3'
> 	
> 	tempname b1 b2 b3
> 	scalar `b1' = `at'[1,4]
> 	scalar `b2' = `at'[1,5]
> 	scalar `b3' = `at'[1,6]
> 	
> 	tempname g11 g12 g13 g14
> 	tempname g21 g22 g23 g24
> 	tempname g31 g32 g33 g34
> 	tempname g41 g42 g43 g44
> 	scalar `g11' = `at'[1,7]
> 	scalar `g12' = `at'[1,8]
> 	scalar `g13' = `at'[1,9]
> 	scalar `g14' = - `g11' - `g12' - `g13'
> 	scalar `g21' = `g12'
> 	scalar `g22' = `at'[1,10]
> 	scalar `g23' = `at'[1,11]
> 	scalar `g24' = - `g21' - `g22' - `g23'
> 	scalar `g31' = `g13'
> 	scalar `g32' = `g23'
> 	scalar `g33' = `at'[1,12]
> 	scalar `g34' = - `g31' - `g32' - `g33'
> 	scalar `g41' = `g14'
> 	scalar `g42' = `g24'
> 	scalar `g43' = `g34'
> 	scalar `g44' = - `g41' - `g42' - `g43'
> 	
> 	quietly {
> 		
> 		tempvar lnpindex
> 		gen double `lnpindex' = 5 + `a1'*`lnp1' + `a2'*`lnp2' +
> `a3'*`lnp3' + `a4'*`lnp4'
> 		forvalues i=1/4 {
> 			forvalues j=1/4 {
> 				replace `lnpindex' = `lnpindex' +
> 0.5*`g`i'`j''*`ln`i''*`ln`j''
> 		    }
> 	    }
> 		
> 		replace `w1' = `a1' + `g11'*`lnp1' + `g12'*`lnp2' +
> `g13'*`lnp3' + `g14'*`lnp4' + `b1'*(`lnexp' - `lnpindex')
> 		replace `w2' = `a2' + `g21'*`lnp1' + `g22'*`lnp2' +
> `g23'*`lnp3' + `g24'*`lnp4' + `b2'*(`lnexp' - `lnpindex')
> 		replace `w3' = `a3' + `g31'*`lnp1' + `g32'*`lnp2' +
> `g33'*`lnp3' + `g34'*`lnp4' + `b3'*(`lnexp' - `lnpindex')
> 	}
> 	
> end
>
> use http://www.stata-press.com/data/r11/food, clear
>
> nlsur aids @ w1 w2 w3 lnp1 lnp2 lnp3 lnp4 lnexp, parameters(a1 a2 a3 b1
> b2 b3 g11 g12 g13 g22 g23 g33) neq(3)

Ivica,

The -syntax- statement of a function evaluator program must accept 'if' conditions, even if you don't specify an 'if' condition in your call to -nlsur-.  Hence, the line

    syntax varlist(min=8 max=8), at(name)

must read

    syntax varlist(min=8 max=8) if, at(name)

Also, this line:

> 				replace `lnpindex' = `lnpindex' +
> 0.5*`g`i'`j''*`ln`i''*`ln`j''

should refer to `lnp`i'' and `lnp`j'', not `ln`i'' and `ln`j''.

To get the maximum-likelihood estimates, you'll need to include the option 'ifgnls' in your -nlsur- command.

Brian P. Poi
Senior Economist
StataCorp LP

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