# Example: Probit Model

The probit model is a flexible and widely-used tool for the analysis of such discrete choice behavior. Choosing between alternatives is omnipresent in everyday life, from the choice of a vehicle for traveling to work over different brands in a supermarket to companies deciding among production plans. Many scientific areas apply the probit model for studying the driving factors behind decision makers’ choices, for example transportation and marketing . Estimating the probit model’s parameters traditionally is performed via maximizing the likelihood function numerically. With rising model complexity however, this approach becomes both computationally expensive and does not guarantee convergence to the global optimum.

## The model formulation

We briefly formulate the probit model and its estimation and refer to Train (2009) and Bhat (2011) for further details. Say that $$N$$ deciders choose among $$J \geq 2$$ alternatives at each of $$T$$ choice occasions. The values for $$J$$ and $$T$$ can be decider-specific, though we do not show this dependence in our notation. Let $$y_{nt} \in \{1,\dots,J\}$$ label the choice of decider $$n$$ at occasion $$t$$. Assume that the choice was rational in the sense that $$y_{nt}$$ yields the highest utility $$U_{nt}$$ for $$n$$ at $$t$$. The probit model defines $U_{nt} = X_{nt} \beta + \epsilon_{nt}$, where $$X_{nt}$$ is a $$J\times P$$-matrix of $$P$$ characteristics for each alternative, $$\beta$$ is a coefficient vector of length $$P$$ and $$\epsilon_{nt} \sim N(0,\Sigma)$$ denotes the vector of jointly normal distributed unobserved influences. We ensure identifiability by taking utility differences and fixing one error-term variance. This implies that instead of $$\Sigma$$, we estimate $$J(J-1)/2-1$$ parameters of a transformed covariance matrix.

The researcher aims to estimate the values for $$b$$ and $$\Sigma$$, most commonly by the maximum likelihood method. Let $$\theta$$ denote the vector of the $$P$$ coefficients of $$b$$ and $$J(J-1)/2-1$$ identified parameters of $$\Sigma$$. Note that the length of $$\theta$$ rises quadratically with $$J$$. The maximum likelihood estimate $$\hat{\theta}$$ is obtained by solving $$$\label{eq:ll} \arg \max_\theta \log \sum_{n,t,j} 1(y_{nt} = j) \int 1(j = \arg \max U_{nt}) \phi(\epsilon_{nt}) d \epsilon_{nt},$$$ where $$1(\cdot)$$ denotes the indicator function and $$\phi(\cdot)$$ the normal density. The integral part of does not have a closed-form expression and hence must be approximated numerically.

## Simulate data from a probit model

The {ino} package provides the function sim_mnp() to simulate data from a probit model. We simulate 10 data sets.

N <- 100
T <- 10
J <- 3
P <- 3
b <- c(1,-1,0.5)
Sigma <- diag(J)
X <- function() {
class <- sample(0:1, 1)
mean <- ifelse(class, 2, -2)
matrix(stats::rnorm(J*P, mean = mean), nrow = J, ncol = P)
}
probit_data <- replicate(10, sim_mnp(
N = N, T = T, J = J, P = P, b = b, Sigma = Sigma, X = X
), simplify = FALSE)

## Setup

The following lines specify the ino object. The likelihood is computed via f_ll_mnp() which is provided via {ino}. Via the global argument, we can specify the true parameter vector thats leads to the global optimum. The mpvs = "data" input specifies that we want to loop over the ten provided data sets.

true <- attr(probit_data[[1]], "true")[-1]
probit_ino <- setup_ino(
f = f_ll_mnp,
npar = 5,
global = true,
data = probit_data,
neg = TRUE,
mpvs = "data",
opt = set_optimizer_nlm(iterlim = 1000)
)

## Random initialization

We initialize runs = 100 times randomly.

probit_ino <- random_initialization(probit_ino, runs = 100)

## Initializing using a subsample

We initialize on a subset of proportion 20% and 50%, which was selected randomly and using kmeans, respectively.

for(how in c("random", "kmeans")) for(prop in c(0.2,0.5)) {
probit_ino <- subset_initialization(
probit_ino, arg = "data", how = how, prop = prop,
ind_ign = 1:3, initialization = random_initialization(runs = 100)
)
}

## Remove runs that did not converge

3 optimization runs reached the iteration limit of 1000 iterations:

library("dplyr", warn.conflicts = FALSE)
summary(probit_ino, "iterations" = "iterations") %>% filter(iterations >= 1000)
#> # A tibble: 3 × 5
#>   .strategy          .time         .optimum .optimizer iterations
#>   <chr>              <drtn>           <dbl> <chr>           <int>
#> 1 random             5290.284 secs     507. stats::nlm       1000
#> 2 subset(kmeans,0.2) 9330.853 secs    1151. stats::nlm       1000
#> 3 subset(kmeans,0.2) 9328.964 secs     975. stats::nlm       1000

We exclude them from further analysis:

ind <- which(summary(probit_ino, "iterations" = "iterations")\$iterations >= 1000)
probit_ino <- clear_ino(probit_ino, which = ind) 

## Comparison

plot(probit_ino, by = ".strategy", time_unit = "mins", nrow = 1)

We see that the subset initialization strategies reduce the computation time significantly, in comparison to the random initialization on the full data set.

## References

Allenby, Greg M., and Peter E. Rossi. 1998. “Marketing Models of Consumer Heterogeneity.” Journal of Econometrics 89 (1): 57–78.
Bhat, Chandra. 2011. “The Maximum Approximate Composite Marginal Likelihood (MACML) Estimation of Multinomial Probit-Based Unordered Response Choice Models.” Transportation Research Part B: Methodological 45.
Bolduc, Denis. 1999. “A Practical Technique to Estimate Multinomial Probit Models in Transportation.” Transportation Research Part B: Methodological 33 (1): 63–79.
Haaijer, Rinus, Michel Wedel, Marco Vriens, and Tom Wansbeek. 1998. “Utility Covariances and Context Effects in Conjoint MNP Models.” Marketing Science 17 (3): 236–52.
Paap, Richard, and Philip Hans Franses. 2000. “A Dynamic Multinomial Probit Model for Brand Choice with Different Long-Run and Short-Run Effects of Marketing-Mix Variables.” Journal of Applied Econometrics 15 (6): 717–44.
Shin, Jungwoo, Chandra R. Bhat, Daehyun You, Venu M. Garikapati, and Ram M. Pendyala. 2015. “Consumer Preferences and Willingness to Pay for Advanced Vehicle Technology Options and Fuel Types.” Transportation Research Part C: Emerging Technologies 60.
Train, Kenneth. 2009. Discrete Choice Methods with Simulation. 2. ed. Cambridge Univ. Press.