st: clogit for discrete choice experiment with multiple choice sets

[Hadji Cortez Jalotjot <jalotjot@yahoo.com>]

Hi!

I 
implemented a discrete choice experiment to model vehicle choice.  In my questionnaire, I presented each respondents with 10 choice experiments 
or choice sets with each choice set having 3 alternatives or choices.

The explanatory variables are the characteristics of the vehicles.  With this, I am fitting a conditional logit model.

In my data set, dummy variables were used to represent the explanatory 
variables.  Since each choice experiment has 3 alternative options, each choice experiment corresponds to 3 rows of observations.  So 10 choice 
experiments per respondent X 3 alternative options per choice 
experiments = 30 rows of observations per respondent. (sample data below shows only 3 choice experiments with 
some of the explanatory variables for respondent 1)


respno        choice_set    choice    var1a    var1b    var1c  .. . ..       none

1                       1                1            1        0        0                        0
1                       1                0            0        0        1                        0
1                       1               0            0        0       0                        1

1                        2              0            0        1        0                        0

1                        2               1            1        0        0                        0

1                        2               0            0        0        0                        1


1                        3                0            0        0        1                        0

1                        3                0            1        0        0                        0

1                        3                1            0        0        0                        1

For clogit to work, I must select a variable that will identify the grouping for which the software will run the analysis.

Now, for this kind of data in which respondents answered multiple choice sets (10 in my case), which should I used as a group?
Is it the respno or choice_set?

I am confused because if I use respno, Stata says multiple positve 
outcomes in a group.  And the predicted probabilities is computed for 
the whole 30 alternative options and not only for the 3 alternative 
options per choice set.

But if I use the choice_set as the grouping and I extend the model to 
include respondent characteristics (e.g. income), I may have problem 
with fixed effects because for example choice_set 1 and choice_set 2 is 
from the same respondent and therefore will have exactly the same 
income.

Any advice is appreciated.

Hadji

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