st: problems with substitutable expression program for nonlinear regression

["Sarah Kristina Reuter" <sarah.kristina.reuter@uni-jena.de>]

Dear Statalisters,

I apologize for a beginner-question, but I have some problems writing a substitutable _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 a specific 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 a substitutable _expression_ program and that you have specified all options that it requires”  and I can’t find the error.

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