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st: RE: re: problem using predictnl after obtaining non-linear estimates


From   Lopa Chakraborti <lchakraborti@arec.umd.edu>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: RE: re: problem using predictnl after obtaining non-linear estimates
Date   Sun, 26 Oct 2008 12:58:01 -0400

Thanks you have pointed me to the right direction. However, I am not using either predict() or xb() functions. Instead, I have:
 predictnl bod1 = ($K2/($K1-$K2))*exp(-$K1*aggthirdfourthadd)*(exp(-$K2*seconddistadd)-exp(-$K1*seconddistadd)). That said my limited knowledge in programming might also explain why I cannot make the connection.
thanks again for your advise

________________________________________
From: owner-statalist@hsphsun2.harvard.edu [owner-statalist@hsphsun2.harvard.edu] On Behalf Of Kit Baum [baum@bc.edu]
Sent: Sunday, October 26, 2008 12:11 PM
To: statalist@hsphsun2.harvard.edu
Subject: st: re: problem using predictnl after obtaining non-linear estimates

<>
Lopa said
I have a general question about predictnl and that is does it evaluate
any function as in any NL function of the parameters and some
explanatory variables or does it only predict dependent variables as
is usually done.



  [R] predictnl -- Obtain nonlinear predictions, standard errors,
etc., after
                        estimation

That help indicates that " pnl_exp is any valid Stata expression" so
that you may calculate any function for each observation, not just the
standard predicted value.

In Stata 10 help,

     When calculating inference-related quantities such as standard
errors, pnl_exp is
     evaluated repeatedly for different values of the model
parameters.  Therefore, think
     of predict() and xb() as a means of substituting for the formula
of the calculation
     and not a means of substituting the value of the calculation that
is obtained when
     the model parameters are set to any specific values.  For example,

         . predict double pred_var, predict_options
         . predictnl newvar = pred_var, se(newvar_se)

     will give standard errors (newvar_se) equal to zero, since once
evaluated, pred_var
     will contain values that are fixed with respect to e(b).  Instead,

         . predictnl newvar = predict(predict_options), se(newvar_se)

     will produce what is intended.



which states that zero standard errors will result. I have no idea
whether it works this way in Stata 8.2.


Kit Baum, Boston College Economics and DIW Berlin
http://ideas.repec.org/e/pba1.html
An Introduction to Modern Econometrics Using Stata:
http://www.stata-press.com/books/imeus.html


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