st: odds ratios and confidence intervals after mlogit with a dummy X

[Jeremy Reynolds <jeremyr@uga.edu>]

Dear Statalist,

I would like to calculate odds ratios and associated confidence
intervals after a multinomial logit that involves an interaction
between a dummy variable and a continuous variable along with other
covariates.  The confidence intervals for the different levels of the
continuous variable are important because the dummy variable has a
negative sign and the interaction a positive sign.  I would like to
determine when the dummy variable decreases and and when it increases
the odds of one outcome in the mlogit.

Drawing on posts to the statalist and Martin Buis' website
(http://www.maartenbuis.nl/publications/interactions.html
), I have written code to calculate the odds ratios associated with
the dummy variable for different levels of the continuous variable
(please see below).

However, there appear to be two potential problems with this approach.

First, nlcom tests whether the odds ratios are different from zero. I
have subtracted 1 from the odds ratio to get the equivalent of testing
whether it is different from 1, but the help file for nlcom makes me
wonder if this procedure is appropriate.