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Re: st: MIXLOGIT: marginal effects


From   Richard Williams <richardwilliams.ndu@gmail.com>
To   statalist@hsphsun2.harvard.edu, statalist@hsphsun2.harvard.edu
Subject   Re: st: MIXLOGIT: marginal effects
Date   Mon, 06 Feb 2012 10:08:14 -0500

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 .


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Richard Williams, Notre Dame Dept of Sociology
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