# Introduction to RprobitB and model formulation

#### 2021-11-12

Welcome to RprobitB! This vignette introduces the R package and defines the underlying model.

## Introdution

With RprobitB1 you can

1. analyze choices made by deciders among a discrete set of alternatives,

2. estimate (latent class) (mixed) (multinomial) probit models in a Bayesian framework,

3. model heterogeneity by approximating any underlying mixing distributions through a mixture of normal distributions,

4. identify latent classes of decision makers.

Run install.packages("RprobitB") in your R console to install the latest version of RprobitB.

## Levels of model complexity

Why the notation (latent class) (mixed) (multinomial) probit model? Because RprobitB can fit probit models of increasing complexity:

• Most basic, modelling the choice between two alternatives (the probit model).

• Considering more than two alternatives leads to the multinomial probit model.

• If we incorporate random effects, the model gets the prefix mixed.

• The most general model is the latent class mixed multinomial probit model, which approximates the mixing distribution through a mixiture of normal distributions.

## The model

Assume that we observe the choices of $$N$$ decision makers which decide between $$J$$ alternatives at each of $$T$$ choice occasions.2 Specific to each decision maker, alternative and choice occasion, we furthermore observe $$P_f+P_r$$ choice attributes that we use to explain the choices. The first $$P_f$$ attributes are connected to fixed coefficients, the other $$P_r$$ attributes to random coefficients following a joint distribution mixed across decision makers.

Person $$n$$’s utility $$\tilde{U}_{ntj}$$ for alternative $$j$$ at choice occasion $$t$$ is modeled as $\begin{equation} \tilde{U}_{ntj} = \tilde{W}_{ntj}'\alpha + \tilde{X}_{ntj}'\beta_n + \tilde{\epsilon}_{ntj} \end{equation}$

for $$n=1,\dots,N$$, $$t=1,\dots,T$$ and $$j=1,\dots,J$$, where

• $$\tilde{W}_{ntj}$$ is a vector of $$P_f$$ characteristics of $$j$$ as faced by $$n$$ at $$t$$ corresponding to the fixed coefficient vector $$\alpha \in {\mathbb R}^{P_f}$$,

• $$\tilde{X}_{ntj}$$ is a vector of $$P_r$$ characteristics of $$j$$ as faced by $$n$$ at $$t$$ corresponding to the random, decision maker-specific coefficient vector $$\beta_n \in {\mathbb R}^{P_r}$$, where $$\beta_n$$ is distributed according to some $$P_r$$-variate distribution $$g_{P_r}$$,

• and $$(\tilde{\epsilon}_{nt:}) = (\tilde{\epsilon}_{nt1},\dots,\tilde{\epsilon}_{ntJ})' \sim \text{MVN}_{J} (0,\tilde{\Sigma})$$ is the models’ error term vector for $$n$$ at $$t$$, which in the probit model is assumed to be multivariate normally distributed with zero mean and covariance matrix $$\tilde{\Sigma}$$.

### Normalization

As is well known, any utility model needs to be normalized with respect to level and scale in order to be identified. Therefore, we consider the transformed model

$\begin{equation} U_{ntj} = W_{ntj}'\alpha + X_{ntj}'\beta_n + \epsilon_{ntj}, \end{equation}$

$$n=1,\dots,N$$, $$t=1,\dots,T$$ and $$j=1,\dots,J-1$$, where (choosing $$J$$ as the reference alternative) $$U_{ntj}=\tilde{U}_{ntj} - \tilde{U}_{ntJ}$$, $$W_{ntj}=\tilde{W}_{ntj}-\tilde{W}_{ntJ}$$, $$X_{ntj}=\tilde{X}_{ntj}-\tilde{X}_{ntJ}$$ and $$\epsilon_{ntj}=\tilde{\epsilon}_{ntj}-\tilde{\epsilon}_{ntJ}$$, where $$(\epsilon_{nt:}) = (\epsilon_{nt1},...,\epsilon_{nt(J-1)})' \sim \text{MVN}_{J-1} (0,\Sigma)$$ and $$\Sigma$$ denotes a covariance matrix with the top-left element restricted to one.3

### Linking the latent utility to the choices

Let $$y_{nt}=j$$ denote the event that decision maker $$n$$ chooses alternative $$j$$ at choice occasion $$t$$. Assuming utility maximizing behavior of the decision makers, the decisions are linked to the utilities via $\begin{equation} y_{nt} = \sum_{j=1}^{J-1} j\cdot 1 \left (U_{ntj}=\max_i U_{nti}>0 \right) + J \cdot 1\left (U_{ntj}<0 ~\text{for all}~j\right), \end{equation}$ where $$1(A)$$ equals $$1$$ if condition $$A$$ is true and $$0$$ else.

### Approximating the mixing distribution

We approximate the mixing distribution $$g_{P_r}$$ for the random coefficients4 $$\beta=(\beta_n)_{n}$$ by a mixture of $$P_r$$-variate normal densities $$\phi_{P_r}$$ with mean vectors $$b=(b_c)_{c}$$ and covariance matrices $$\Omega=(\Omega_c)_{c}$$ using $$C$$ components, i.e. $\begin{equation} \beta_n\mid b,\Omega \sim \sum_{c=1}^{C} s_c \phi_{P_r} (\cdot \mid b_c,\Omega_c), \end{equation}$ where $$(s_c)_{c}$$ are weights satisfying $$0 < s_c\leq 1$$ for $$c=1,\dots,C$$ and $$\sum_c s_c=1$$.

### Interpreting the latent classes

One interpretation of the latent class model is obtained by introducing variables $$z=(z_n)_n$$ allocating each decision maker $$n$$ to class $$c$$ with probability $$s_c$$, i.e. $\begin{equation} \text{Prob}(z_n=c)=s_c \quad \text{and} \quad \beta_n \mid z,b,\Omega \sim \phi_{P_r}(\cdot \mid b_{z_n},\Omega_{z_n}). \end{equation}$

We call this model the latent class mixed multinomial probit model.5

1. The package name RprobitB is a portmanteau, combining R (the programming language), probit (the model class) and B (for Bayes, the estimation method).↩︎

2. For notational simplicity, the number of choice occasions $$T$$ is assumed to be the same for each decision maker here. However, RprobitB allows for a different number of choice occasions for each decision maker.↩︎

3. RprobitB provides an alternative to fixing an error term variance in order to normalize with respect to scale by fixing an element of $$\alpha$$.↩︎

4. We use the abbreviation $$(\beta_n)_n$$ as a shortcut to $$(\beta_n)_{n =1,...,N}$$ the collection of vectors $$\beta_n,n=1,...,N$$.↩︎

5. Note that the model collapses to the (normally) mixed multinomial probit model if $$P_r>0$$ and $$C=1$$, to the multinomial probit model if $$P_r=0$$ and to the basic probit model if additionally $$J=2$$.↩︎