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st: Using mfx after clogit -- compare marginals to what?


From   paul jacobs <pauldjacobs@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: Using mfx after clogit -- compare marginals to what?
Date   Tue, 4 Feb 2014 11:10:32 -0500

I am trying to interpret the marginal effects of several covariates
after running a conditional logit model.  My question concerns how to
choose the correct reference point for comparing the size of these
marginal effects.

I first run a conditional logit model in stata (using the command,
clogit) and then use the following postestimation command to retrieve
marginal effects (which sets the fixed effects, which are person-level
in my case, to zero):

mfx, predict (pu0)

The rationale for choosing that command is explained here:
http://www.stata.com/support/faqs/statistics/predict-option-unsuitable/

My question is: how do I interpret the size of the marginal effects?
On average, let's assume the people in my sample have 33.3 choices
(each person chooses one and only one of the average of 33.3 choices),
implying a mean probability across the sample of choosing any one
outcome of ~3%.  However, when the marginal effects are being
calculated (and the individual fixed effects are suppressed), the mean
predicted probability across the sample is 90%, which is shown in the
output for mfx with the pu0 option.

Let's assume a particular covariate's marginal effect is a 1
percentage point increase in the probability of choosing an outcome.
It matters greatly whether 3% or 90% is my reference point for
comparing the relative size of that marginal effect. 3% seems like the
better comparison because it is the actual probability of choosing an
option across the sample but 90% is the mean predicted value from the
mfx command which produced the marginal effects. Which one is the
appropriate comparison and why?

Thank you Stata listserve members!
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