 Choice model simulation

Although the concept choice models is widely understood and practiced these days, it is often difficult to acquire handson knowledge in simulating choice models. While many stat packages provide useful tools to simulate, researchers attempting to test and simulate new choice models with data often encounter problems from as simple as scaling parameter to misspecification. This article goes beyond simply defining discrete choice models. Rather, it aims at providing a comprehensive overview of how to simulate such models in computer.
Contents
Defining choice set
When a researcher has some consumer choice data in his/her hand and tries to construct a choice model and simulate it against the data, he/she needs to first define a choice set. A Choice Set in discrete choice models is defined to be finite, exhaustive, and mutually exclusive. For instance, consider households' choice of how many laptops to own. The researcher can define the choice set depending on the nature of the data and the interpretation they wish to draw, as long as it satisfies three properties mentioned above. Some examples of choice sets that meet the categories are the following:
 0 , 1, More than 1 laptop
 0 , 1 , 2 , More than 2 laptops
 Less than 2 , 2 , 3 , 4 , More than 4 laptops
Defining consumer utility
Suppose a student is trying to decide which pub he/she should go for a beer after his/her last final exam. Suppose there are two pubs in the town of the college: an Irish pub and an American pub. The researcher wishes to predict which pub he/she will choose based on the price (P) of beer and the distance (D) to each pub, assuming they are known to the researcher. Then, the consumer utilities for choosing the Irish pub and the American pub can be defined:
 (1)
 (2)
where captures unobserved variables that affect consumer utilities.
Defining choice probabilities
Once the consumer utilities have been specified, the researcher can derive choice probabilities. Namely, the probability of the student choosing the Irish pub over the American pub is
Denoting the observed portion of the utility function as V,
 P_{i} = Pr(ε_{i} − ε_{a} > V_{a} − V_{i}) (3)
In the end, discrete choice modeling comes down to specifying the distribution of (or \varepsilon_i  \varepsilon_a) and solving the integral over the range of to calculate P_{i}. Extending this to more general situations with
 N consumers (n = 1, 2, …, N),
 J choices of consumption (j = 1, 2, … , J),
The choice probability of consumer n choosing j can be written as
 P_{nj} = Pr(U_{nj} > U_{ni}) (4)
for all i other than j
Identification
1. What's irrelevant
From equation (4), it's obvious that P_{nj} does not change as long as the inequality in the probability argument on the right side stays the same. In other words, adding or multiplying by a constant to all U_{n1}...U_{nJ} does not change the choice probably, thus no change in interpretation.
2. Alternativespecific constants
Unlike adding a constant to all the utilities, adding alternativespecific constants does alter the choice probabilities. Suppose alternativespecific constants C_{i} and C_{a} are added to (1) and (2):
Then, depending on the value of the estimated alternativespecific constants, the choice probability may change. Also if we write the choice probability in the format of (3),
 P_{i} = Pr(ε_{i} − ε_{a} > (C_{a} − C_{i}) + αP_{a} − αP_{i} + βD_{a} − βD_{i})
only the difference between C_{a}andC_{i} affects the choice probability (i.e. our estimation can only identify the difference). So it's convenient to normalize all the alternativespecific constants to one of the alternatives. If we normalize to C_{i}, then we estimate the following model:
When there are more than 2 choices in the choice set, we can pick any choice i and normalize the alternativespecific constants to that choice by subtracting C_{i} from all other alternativespecific constants.
3. Sociodemographic variables
In deciding between the Irish pub and the American pub, if the researcher has access to additional sociodemographic variables such as income, they can enter the consumer utility equation in various ways. Denote the student's income as Y. If the researcher believes that the income affects the utility linearly, then
If the researcher believes that the sociodemographic variable interacts with other variable such as price, then the utility can be written as
General models
As mentioned earlier, calculation and justification of choice probabilities rely on the properties of the error (i.e. the unobservables) distribution function the researcher specifies. Here is the quick overview of frequently used models that each differs in specification
1. Logit:
 Assumes unoberseved factors have the same variance with zero correlation across alternatives.
 iid extreme value unobserved factors
 The cumulative distribution of difference in extreme values is Logistics function
 Logistics function has a closed form solution => No simulation necessary.
2. GEV (Generalized extreme value distribution)
 Allows correlation in unobserved factors across alternatives.
 iid extreme value unobserved factors
 The cumulative distribution of difference in extreme values is Logistics function
 Logistics function has a closed form solution => No simulation necessary.
3. Probit
 Unobeserved factors have a jointly normal distribution.
 No closed form for the cumulative distribution of normal distribution. Simulation necessary.
4. Mixed logit
 Allows any distribution in unobserved factors
 No closed form for the cumulative distribution of normal distribution. Simulation necessary.
References
 A Nevo (2000). "Practitioners Guide to Estimation of Random Coefficients Logit Models of Demand," Journal of Economics & Management Strategy, 9(4), 513–548
 Kenneth E. Train, " Discrete Choice Methods with Simulation", Massachusetts: Cambridge University Press, 2003.
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