Julia,
-clogit- is used for matched data. I guess your ID variable identifies the
matching pair and that you are using this variable in the -group- option
-group(id))-. I think you should have only one variable for transportation
cost. So, for the first subject of the pair cost==120 and for the second
subject cost==20. In the way you have your data now, the outcome variable
(mode) is completely identifiable by the independent variables (all choice=1
have public cost=0 and all choice=0 have private cost=0). I don't think you
can get meaningful results in this way.
Leonelo Bautista
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Julia Gamas
Sent: Tuesday, January 18, 2005 9:25 AM
To: [email protected]
Cc: [email protected]
Subject: st: clogit: two questions
Dear all,
I have two questions about clogit: one, about whether or not I "fed" the
data
correctly to Stata, the second, about the sign on my coefficients. I do
appologize if there is a simple answer that I overlooked.
1. I've "fed" data that is alternative specific to find the choice=1 of
using
private transportation versus choice=0 of using public transit. I have
sociodemographic variables and generic variables too, but my concern is with
the alternative specific variable of mode cost. The format I used was the
following for all ID's, can anybody tell me if this is wrong and if so, how
to
fix it (I couldn't find more information in the Stata manual that might
help).
Id Mode Choice Car-Cost Public-transit-Cost
32 1 1 120 0
32 2 0 0 20
Did I do the right thing by putting in the cost of using a car when the mode
is
a car and zero in car cost when the mode is public?
2. I obtain a POSITIVE coefficient from public transit cost. My
interpretation
is that the more public tranist costs, the less we are likely to want to use
it
and we may substitute to a car. I interpret this assuming that the model is
estimating V1-V2 (utility of driving - utility of using public transit) as
in
the logit equations. This would imply that the coefficient for public
transit
cost is negative in the utility of using public transit, but changes sign to
positive when we subtract that utility from utility of driving. I wanted to
check with you if this interpretation is correct, or do I, in fact, have the
wrong sign?
Any help you can give me would be immensely appreciated.
Sincerely,
Julia A. Gamas
Mexico City Project, EAPS
77 Massachusetts Avenue 54-1823
Cambridge, MA 02139
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