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Re: st: computing elasticities after using lpoly


From   Arka Roy Chaudhuri <gabuisi@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: computing elasticities after using lpoly
Date   Sat, 6 Oct 2012 03:07:49 -0700

Thank you Nick for your advice. However my point in using lpoly is
precisely its flexibility. But at the same time I want a measure of
the slope of the curve that I get from using lpoly at different points
of the curve-so I am not actually looking for one specific
number.Rather I am trying to find the slope at different values of my
independent variable-so that I can get an idea of the elasticities at
different parts of the expenditure distribution. Basically I am trying
to implement both Fig 2 and Fig 3 in the following paper (albeit in my
context):Deaton, Angus S. and Subramanian, Shankar. "The Demand for
Food  and Calories," Journal of Political Economy, Feb.  1996,104(1),
pp. 133-62

On Sat, Oct 6, 2012 at 2:19 AM, Nick Cox <njcoxstata@gmail.com> wrote:
> It seems to me that the whole point of -lpoly- is to be flexible about
> modelling a relationship. It has absolutely no sense of any idea of an
> overall slope. If you want a number for the slope, -lpoly- is useless.
> If you want an independent view of how far the relationship after some
> smoothing really is (e.g.) linear or monotonic, then -lpoly- can be
> useful, mostly by providing a graph. For what you want, any
> appropriate regression method will be better, such as -regress- or
> -qreg-.
>
> Nick
>
> On Sat, Oct 6, 2012 at 10:06 AM, Arka Roy Chaudhuri <gabuisi@gmail.com> wrote:
>
>> I am using Stata 11 on a Windows 7 machine. I am using lpoly to
>> estimate nonparametric regressions of the form:
>>
>> log(y)=f(log(x)) + u
>>
>> where y= per capita expenditure on food
>> x= total per capita expenditure
>>
>> Using lpoly, I can get a graph of per capita expenditure on food
>> against total per capita expenditure. However I am also interested in
>> obtaining expenditure elasticities of percapita expenditure on food at
>> different levels of  total per capita expenditure i.e in my context
>> d(log(y)/d(log(x)) where x and y are as earlier defined.The problem is
>> that lpoly does not give estimates of any slope coefficients so I am
>> at a loss on how to compute these elasticities. I would really
>> appreciate if anybody could give me advice in this regard.
>>
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