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st: RE: Calculate confidence intervals for log-log models


From   "Seed, Paul" <paul.seed@kcl.ac.uk>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Calculate confidence intervals for log-log models
Date   Wed, 6 Mar 2013 10:26:14 +0000

The same result can be got in a simpler way.

Consider a well-known example:
*********** Start of Stata code ***********
clear
sysuse auto
gen ln_price = ln(price)
gen ln_mpg = ln(mpg)

regress ln_mpg ln_price
* effect of a 10% increase in price on mpg.
lincom `=ln(1.1)'*ln_price , eform
* The mpg falls on average by 3.4% to 96.6% of the original value.

* Alternative method
nlcom 1.1^_b[ln_price]
* The same confidence interval; but the test (t in particular) 
* no longer agrees with that given by the regression. 


***********  End of Stata code  ***********



*******************************************************************

Date: Tue, 5 Mar 2013 20:58:57 +0000
From: Jason Ferris <j.ferris@uq.edu.au>
Subject: st: RE: Calculate confidence intervals for log-log models

Hi  Nick and Rich,

Nick - I was going to run a series of 'changes' in the original variable (as you suggested) by dividing the original variable by a series of constants (representing the percentage value changes) that I want.  I thought I could get this directly by using lincom - but didn't appreciate it was only about 'linear combinations'.

Rich - thank you for suggestion - this gives me exactly what I am looking for.

- ------------------------------------------------------------------------------
         lny |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
- -------------+----------------------------------------------------------------
         lnx |   2.105722   .0817271    25.77   0.000     1.942802    2.268642
       _cons |  -3.032146   .4274653    -7.09   0.000    -3.884282   -2.180009
- ------------------------------------------------------------------------------

. nlcom 1.1^_b[lnx]

       _nl_1:  1.1^_b[lnx]

- ------------------------------------------------------------------------------
         lny |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
- -------------+----------------------------------------------------------------
       _nl_1 |   1.222254   .0095207   128.38   0.000     1.203275    1.241233
- ------------------------------------------------------------------------------

Cheers,

Jason 


***************************************************
Paul T Seed, Senior Lecturer in Medical Statistics, 
Division of Women's Health, King's College London
Women's Health Academic Centre, King's Health Partners 
(+44) (0) 20 7188 3642.



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