st: ML program in Stata 12.1 for a nonlinear equation of the form Y=F(x)*G(z)

[Antony Chapoto <achapoto@gmail.com>]

I would like your assistance to write an ML program in Stata 12.1 for
the following nonlinear model:


 Y=F(x)*G(z)


where F(x)=a1X1 +a2X2 +a3X3 +a4X4  and G(z) =exp[-(b0+b1Z1+b2Z2 +b3Z3
+b4Z4 +b5Z5)^2] where a1 – a4,  b0, b1-b5 are parameters to be
estimated  and X, Z are explanatory variables


 Essentially, I would like the effect of x on Y to be adjusted by
G(z).  What is the correct  ML program assuming that we have normally
distributed errors.  The expanded model is as follows:


Y=(a1X1 +a2X2 +a3X3 +a4X4)*exp[-(b0+b1Z1+b2Z2 +b3Z3 +b4Z4 +b5Z5)^2] +e


I attempted to write a program following an example at
http://www.stata.com/support/faqs/statistics/nonlinear-models-using-ml/
  but failed to come up with the t " ml model " for the program to
run.  Your assistance is greatly appreciated.



capture program drop myprog

program myprog

 version 12.1

  args lnf b1x a1x sigma

  tempvar res

  qui gen double `res' = $ML_y1 – (`b1x')*exp(-(`a1x')^2))

  qui replace `lnf' = -0.5*ln(2*_pi)-ln(`sigma')-0.5*`res'^2/`sigma'^2



Best regards,



Antony

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