[Limdep Nlogit List] Question on socio-demographic characteristics and the MNL model

[Oliver Ehrlich]

Dear all,

I have a question concerning the inclusion of socio-demographic
characteristics (SDCs) in discrete choice MNL models:

Let's say I want to model 1) the influence of perceived sales channel
quality (service quality, convenience, relative product price) as stated
by consumers and 2) the influence of SDC "age" of consumers on sales
channel choice (store, internet, hotline) with a classical MNL model.

In NLOGIT 4.0, I would use the NLOGIT command with
lhs=choice;
choices=store,internet,hotline;
model:
U(store)=asc_s+par_qual*quality+par_conv*convenience+par_price*price+par_age*age/
U(internet)=asc_i+par_qual*quality+par_conv*convenience+par_price*price+par_age*age/
U(hotline)=par_qual*quality+par_conv*convenience+par_price*price $

The question for me now is: is there any meaningful interpretation of the
parameter "par_age" for the SDC "age"? For the channel attributes, the
interpretation is of course straightforward (e.g., positive parameter
"par_qual" = the higher service quality in a channel, the higher its
channel choice probability); but as age is by definition invariant across
the alternative channels, the interpretation of a positive/negative
parameter par_age would be "the older/younger a consumer, the higher his
overall channel choice probability" - which doesn't make too much sense...

I have also tried alternative-specific parameters for the SDC; however,
the sign of these parameters turned out to be either always positive or
always negative across alternatives, which leads to the same meaningless
interpretation as in the generic parameter case.

Against this background, I can't currently think of a setup where a SDC
would lead to a meaningful interpretation in my model setting... Maybe
some of you have an idea how SDCs could be included in the model in a
meaningful way - or if they in general should be left out completely in
such a model setting.

Thanks in advance for your help,
regards

Oliver