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Re: st: Reconcile Log Transformed with Untransformed Results


From   Austin Nichols <austinnichols@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Reconcile Log Transformed with Untransformed Results
Date   Tue, 16 Feb 2010 12:11:53 -0500

Erasmo Giambona <e.giambona@gmail.com>:
As I already pointed out, I doubt your estimates correspond to any
well-defined percentage point change.  Perhaps you can give us a
better sense of the distributions of the untransformed y and x (and
what they measure and in what units), and what the scatterplot of y
against x looks like.  You may also prefer to state your effects in
terms of standard deviations rather than the interquartile range.

On Tue, Feb 16, 2010 at 9:39 AM, Erasmo Giambona <e.giambona@gmail.com> wrote:
> Thanks Maarten. In this example, OLS and GLM give very similar
> econimic effects. In fact, 74 cents for the OLS is really 9.52%
> relative to the mean wage of 7.77. This 9.52% is very much in line
> with the 9.7% found with GLM. In my case, the coeff. on X for the OLS
> is 0.0064. Relative to the mean for the LHS variable of 0.02. This is
> an economic effect of about 28%. With the GLS, using exactly your
> code, X gets a coefficient of 2.025 or a 102.5% increase in Y. Or
> perhaps, I am misinterpreting this coefficient.
>
> Thanks,
>
> Erasmo
>
> On Mon, Feb 15, 2010 at 9:22 AM, Maarten buis <maartenbuis@yahoo.co.uk> wrote:
>> --- On Sun, 14/2/10, Erasmo Giambona wrote:
>>> I ran the regressions with both RHS and LHS untransformed
>>> using both OLS and GLM with link(log). With the OLS the
>>> coeff on X is 0.006 while with the GLM the coefficient is
>>> 0.700. I find a bit hard to intepret the GLM coefficient.
>>
>> Consider the example below:
>>
>> *--------------- begin example -----------------
>> sysuse nlsw88, clear
>> gen byte baseline =1
>>
>> reg wage grade
>> glm wage grade baseline,  ///
>>    link(log) eform nocons
>> *--------------- end example --------------------
>>
>>
>> The -regress- results are interpreted as follows:
>> People without education can expect a wage of
>> -1.96 dollars an hour (substantively we know that
>> people hardly ever pay for the privelege to work,
>> so this is a sign of bad model fit), and they get
>> 74 cents an hour more of every additional year of
>> education.
>>
>> The -glm- results are interpreted as follows:
>> People without education can expect a wage of
>> 2.25 dollars an hour, and for every additional
>> year of education they can expect an increase
>> of 9.7%.
>>
>> Hope this helps,
>> Maarten

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