Hi all,
I'm trying to make sure that my interpretation of my
clogit coefficient results is correct. My basic
question is: do the signs reported in the clogit refer
to the impact that a given variable has on overal
utility OR on the utility of one choice as compared to
the other?
I have two choices: "used private transportation" or
"used public transit". The individual specific
variables have been "dummied" such as:
genderprivate,ageprivate incomeprivate.
I also have a choice specific variable: "trip
duration".
1. My first question is if I get a positive
coefficient on my choice dependent variable (trip
time), does this mean that the longer the trip, the
more likely I am to choose private transportation?
But in essence, shouldn't this coefficient be negative
(as trip time takes away from utility in general)?
2. Furthermore, if (following Ben-Akiva 1996) I
assume that trip times have a different quality for
public than for private transit, then should the
coefficient for publictime be positive (the more trip
time on public transit, the more likely I am to
drive)? And should the coefficient on private trip
time be negative? Or should both be negative?
Examples of 1 follows:
1. Variable "duracion" is the trip duration.
. clogit choice private incpppriv pphhpriv headpriv
publiccost privatecost duracion, group (numob)
note: 393 groups (393 obs) dropped due to all positive
or
all negative outcomes.
Iteration 0: log likelihood = -3513.9953
Iteration 1: log likelihood = -26.537414
Iteration 2: log likelihood = 0
Iteration 3: log likelihood = 0
Conditional (fixed-effects) logistic regression
Number of obs = 83102
LR
chi2(7) = 57601.92
Prob
> chi2 = 0.0000
Log likelihood = 0
Pseudo R2 = 1.0000
------------------------------------------------------------------------------
choice | Coef. Std. Err. z P>|z|
[95% Conf. Interval]
-------------+----------------------------------------------------------------
private | -41.57196 1.80e+07 -0.00 1.000
-3.53e+07 3.53e+07
incpppriv | -.0033245 68243.68 -0.00 1.000
-133755.2 133755.2
pphhpriv | .0100637 2625669 0.00 1.000
-5146218 5146218
headpriv | -.3225972 1.02e+07 -0.00 1.000
-2.00e+07 2.00e+07
publiccost | -82.60774 1.31e+07 -0.00 1.000
-2.57e+07 2.57e+07
privatecost | -.0002418 270797.4 -0.00 1.000
-530753.1 530753.1
duracion | -.0072196 192486.6 -0.00 1.000
-377266.9 377266.9
------------------------------------------------------------------------------
Any help will be appreciated.
Sincerely,
Julia A. Gamas, Ph.D.
Mario Molina Center for Energy and Strategic Studies
END OF CLOGIT INTERPRETATION QUESTION **************
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