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From |
"Brian P. Poi" <bpoi@stata.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: problems with substitutable expression program for nonlinear regression |

Date |
Fri, 04 Nov 2011 09:16:38 -0500 |

Sarah Kristina Reuter wrote:

Dear Statalisters,I apologize for a beginner-question, but I have some problems writing asubstitutable expression program for nonlinear regression. The model is:y = y0 * (1-x^q)y and x are given, q is to be estimated and y0 is the y-value of aspecific data-point, namely the smallest x.The problems I have are - “explaining” Stata y0 and- Stata returns the following error message: “verify that nlreg is asubstitutable expression program and that you have specified all optionsthat it requires” and I can’t find the error.

Here is a simple version that does work: program nlreg, rclass version 10 syntax varlist(min=2 max=2) [aw fw iw] [if] [in] marksample touse local lhs: word 1 of `varlist' local rhs: word 2 of `varlist' *specifying y0, even to me that looks complicated tempname a su `rhs' if `touse', meanonly scalar `a' = r(min) su `lhs' if `rhs' == `a', meanonly scalar y0 = r(min) return local eq "`lhs' = y0*(1-`rhs'^{gamma})" return local title "`lhs' = y0*(1-`rhs'^gamma)" end nl reg: y x Here I did not make y0 a temporary scalar. Temporary scalars disappear as soon as a program exits, so -nl- would not know what `y0' was referring to. My program assumes that there is not a variable in your dataset also named y0. One solution to that would be to use a temporary scalar for y0, but use Stata's `=...' notation to have Stata substitute the numerical value for y0 before continuing, like this: *specifying y0, even to me that looks complicated tempname a y0 su `rhs' if `touse', meanonly scalar `a' = r(min) su `lhs' if `rhs' == `a', meanonly scalar `y0' = r(min) return local eq "`lhs' = `=`y0''*(1-`rhs'^{gamma})" return local title "`lhs' = `=y0''*(1-`rhs'^gamma)" In the first version of my program, the title just above the coefficient table will read y = y0*(1-x^gamma) while in the second version it will read y = ###########*(1-x^gamma) where ########### is whatever number y0 happens to be for your dataset. You can set the title macro however you want; it just controls that title you see in the output. Brian P. Poi Senior Economist StataCorp LP

Here is my program: program nlreg, rclass version 10 syntax varlist(min=2 max=2) [aw fw iw] [if] [in] tempvar touse mark `touse' `if' `in' local lhs: word 1 of `varlist' local rhs: word 2 of `varlist' *specifying y0, even to me that looks complicated tempname a b su `rhs' if `touse’, meanonly scalar `a' = r(min) gen `b’=`lhs’ if `rhs’==`a’ su `b’, meanonly scalar `y0’=r(min) return local eq "`lhs' = `y0'*(1-`rhs'^{gamma})" return local title "`lhs' = `y0'*(1-`rhs'^gamma)" end So happy for any suggestions! Thanks! Sarah

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**References**:**st: problems with substitutable expression program for nonlinear regression***From:*"Sarah Kristina Reuter" <sarah.kristina.reuter@uni-jena.de>

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