st: Discrete choice estimation

["Data Analytics Corp." <walt@dataanalyticscorp.com>]

Hi Stata,

I want to estimate a stated preference discrete choice model in Stata 
(Stata 11 on Windows).  Let's say it's just a simple binary model with 
alternatives A and B.  For a forced choice problem, I don't have a 
problem doing the estimation.  I would use clogit with group(caseID) 
where caseID specifies each alternative for each choice set for each 
person.  My problem is that a Neither is included.  The original design 
matrix only has the two alternatives for the forced choice.  The Neither 
was added in the questionnaire as it should be.  Suppose my data consist 
of three independent variables (Color, Shape, Price), all categorical.  
It seems that the data for the first two choice sets, therefore, should be
        RespondentID    Set     caseid    choice        Color       
Shape       Price
        1                         1           1              0        
   red         square       low
        1                         1           1              1        
    blue        round        high
        1                         1           1              0        
   none        none        none
        1                         2           2              0       
    blue        round        high
        1                         2           2              0      
     blue        round        low
        1                         2           2              1        
    none       none          none
where the third and sixth rows reflects the Neither.  Only rows 1,2,4,5 
are from the design matrix where rows 1 and 2 are for alternative A and 
the second for B; similarly for 4 and 5.  The last is added afterward to 
reflect Neither.  Is this correct?  The Stata command would be
        xi: clogit choice i.Color i.Shape i.Price, group(caseID)

How does Stata handle the estimation?  Is the Neither alternative 
omitted (I suspect so)?  If so, is the effect of Neither 0?
Thanks,

Walt

________________________

Walter R. Paczkowski, Ph.D.
Data Analytics Corp.
44 Hamilton Lane
Plainsboro, NJ 08536
________________________
(V) 609-936-8999
(F) 609-936-3733
walt@dataanalyticscorp.com
www.dataanalyticscorp.com

_____________________________________________________


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