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st: FW: Elasticity using nlcom

From   "Martin Weiss" <>
To   <>
Subject   st: FW: Elasticity using nlcom
Date   Mon, 7 Jun 2010 21:38:40 +0200


So Henry sent this query to me privately. I am not sure what his -global-s
"a" and "b" contain, but from his second example, I think they represent
scalars. Henry should think about the point of using -nlcom-: "nlcom
computes point estimates, standard errors, test statistics, significance
levels, and confidence intervals for (possibly) nonlinear combinations of
parameter estimates after any Stata estimation command." (see the help file)

So when Henry feeds -nlcom- his call
-, Stata sees variable names instead of parameter estimates, and the error
message makes the problem abundantly clear. Let me replicate:

sysuse auto, clear
reg price weight length
nlcom (myratio: weight/length )

His second -nlcom- call is:
-nlcom((-.0093/35)*((36/.33)-$b*(3.7-$a*14.4))*8.89)- which Stata evaluates
to -nlcom
-. This, of course, does not contain a reference to a parameter estimate,
either, so Stata returns a scalar, and no other values. Where is a standard
error supposed to come from for a _scalar_? 

All of which makes me think that Henry really wants something else, and my
money is on -predictnl-.


-----Original Message-----
Sent: Montag, 7. Juni 2010 20:46
Subject: Elasticity using nlcom

Dear Martin,

I am doing estimation using the AIDS model. In order to estimate elasticity,
I used nlcom command on an elasticity function I derived through
differentiation as follows:


The exogenous variables are CPI, CIF and the lags of import and domestic

However, stata complained that reference was made to X rather
than _b[X] as follows:

contains  reference to X rather than _b[X]

When evaluated at their means I got a value but without standard errors and
p-value as below:




P>|t|      Coef.                              [95% Conf. Interval]
                       Std. Err.      t
_nl_1 -.255856          .              .                   .     


I need to know what is going on and what I must do to get values for the
standard errors,  t-stat and conf. interval.

Thank you,



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