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Re: st: MIXLOGIT: marginal effects


From   Arne Risa Hole <arnehole@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: MIXLOGIT: marginal effects
Date   Mon, 6 Feb 2012 17:04:41 +0000

Thanks Richard, these are all good points. To be slightly pedantic I
did say 1 _unit_: height, for example, can be measured in metres or
centimetres (or feet and inches) and the procedure I outlined will
clearly be more sensible if the latter is used.

Arne

On 6 February 2012 15:08, Richard Williams
<richardwilliams.ndu@gmail.com> wrote:
> At 08:18 AM 2/6/2012, Arne Risa Hole wrote:
>>
>> Dear Davide,
>>
>> I haven't created a module for calculating marginal effects after
>> -mixlogit- but this can be done using simulation as follows:
>>
>> 1) Use -mixlpred- to calculate predicted probabilities in the base
>> scenario
>>
>> 2) Increase the relevant variable by 1 unit for all individuals in the
>> sample
>>
>> 3) Use -mixlpred- to calculate predicted probabilities in the
>> alternative scenario
>>
>> 4) Calculate the difference between the predicted probabilities in 1)
>> and 3) and average this difference over individuals. This gives you an
>> estimate of the marginal effect.
>
>
> I've never used -mixlogit- so Arne can correct me if I am wrong, but I would
> modify the advise as follows:
>
> The "increase by one" part may or may not work well; it depends on the
> variable scaling. It might work well if the variable ranges between 0 and
> 10,000 and not work well if it ranges between 0 and .10. It might be better
> to increase by, say, .001, and divide the difference by .001, i.e. divide
> the change in Y by the change in X.
>
> P. 353 of "Microeconomics Using Stata, Revised edition" recommends (p. 353,
> section 10.6.10) using a change equal to the standard deviation of the
> regressor divided by 1,000. The book can be bought from the Stata bookstore
> and the code can be downloaded for free.
>
> Also, the procedure will differ for continuous and discrete variables. With
> discrete, you would first set the value to 0, then set the value to 1, and
> compute the difference. For an example see slides 28-30 of
> http://www.nd.edu/~rwilliam/stats/Margins01.pdf .
>
>
> -------------------------------------------
> Richard Williams, Notre Dame Dept of Sociology
> OFFICE: (574)631-6668, (574)631-6463
> HOME:   (574)289-5227
> EMAIL:  Richard.A.Williams.5@ND.Edu
> WWW:    http://www.nd.edu/~rwilliam
>
>
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