[Limdep Nlogit List] Help with clogit model

[Achilleas Vassilopoulos]

Dear Martin,

When respondents choose between air, bus, train etc., we are talking about a
labeled experiment whereas yours is an unlabeled one. You can easily
estimate unlabeled choice experiments using Nlogit by just assigning random
names in each of your alternatives (e.g A, B, C, D). Note that although A,
B, C and D will be different for every respondent, since you do not include
a constant term in your model (i.e "ONE" in your Rhs variables) the results
you get are the generic parameter estimates of the utility associated with
each attribute or attribute level (i.e price, video, setup_cost,
enhance_service).

What I'm not sure about is how have you ended up with these 16 alternatives
(combinations as you refer to)? Where they a result of  a full or fractional
factorial design or you have assigned the attributes and the attributes'
level of each alternative  arbitrarily or by experience?  In the last case,
there is no possible way to estimate such a model in an efficient and
unbiased manner since your Rhs variables are probably highly correlated and
the parameters you'll get (if any) will be completely uninformative. At
last, instead of  just splitting all 16 possible alternatives in 4 choice
sets to reduce the size of cards, you need to employ blocking techniques in
your experimental design. 

I hope that helps,
_____________ - _______________

Achilleas Vassilopoulos

Agricultural University of Athens,
Dept. of Agricultural Economics and Rural Development,
Lab. of Political Economy and European Integration.
Iera Odos 75, 11855, Athens, Greece

Tel: (+30) 210-5294726
Fax: (+30) 2105294786



-----Original Message-----
From: limdep-bounces at limdep.itls.usyd.edu.au
[mailto:limdep-bounces at limdep.itls.usyd.edu.au] On Behalf Of Manit
Satitsamitpong
Sent: Wednesday, April 28, 2010 3:21 PM
To: limdep at limdep.itls.usyd.edu.au
Subject: [Limdep Nlogit List] Help with clogit model

Dear All,

I am very new in this area. I really hope someone could help me with the
command or concept about the choice model that I am facing right now.


Suppose I would like to estimate customer preferences and the relative
importance of service factors of IPTV such as price per month (range of
price), video on demand feature (high or low), setup cost (high or low),
enhanced service (yes or no),and other 2 factors.  Then I created say 16
profiles of combinations of those factors (conjoint questions)  for
respondents to answer.  I understood that if I asked respondents to rank all
those 16 profiles (combinations) then I can use SPSS to run the conjoint
analysis and then I can calculate the importance of each factor.

My question here is what if I break down those 16 profiles into 4 choices
sets with each set contains 4 profiles (combinations) and then ask
respondents to choose the best choice (alternative) among each choice set.
 In this case, I don't think I can use clogit command to do so.  Because
clogit command asks me to provide the choice list (specific names of each
choice) but in this case I don't have a specific name for each combination
of choice set.

clogit   ;lhs = choice
            ;Rhs = price, video, setup_cost, enhance_service
           ;Choices = ????
$

Here I don't know what to use as the parameters for Choices since all those
16 combinations are different, unlike the transportation mode (air, bus,
car, train) given in Professor Greene's book.  Is there any way for me to
use LIMDEP or other software to run this data given the data that i have
(choice data instead of ranking data).   Thank you very much.  I am really
appreciated your time for reading this mail.


Best regards,
Manit Satitsamitpong
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