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# st: problems with substitutable expression program for nonlinear regression

 From "Sarah Kristina Reuter" To statalist@hsphsun2.harvard.edu Subject st: problems with substitutable expression program for nonlinear regression Date Fri, 04 Nov 2011 13:46:06 +0100

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