Friday, March 17, 2017

Aggregate random coefficients logit—a generative approach

This post illustrates how to fit aggregate random coefficient logit models (often called BLP) in Stan, using Bayesian techniques. It’s far easier to learn and implement than the BLP algorithm, and has the benefits of being robust to mis-measurement of market shares, and giving limited-sample posterior uncertainty of all parameters (and demand shocks). This comes at the cost of an additional assumption: we employ a parametric model of how unobserved product-market demand shocks affect prices, thereby explicitly modeling the endogeneity between prices and observed consumer choices, whose biasing effect on estimates classical BLP uses instrumental variables to resolve. Because we specify the full generative model, instrumental variables are not strictly needed (though they can certainly still be incorporated).

Introduction

A common problem in applied economics, especially in industrial organization or marketing, is that a decision maker wants a good model of how individual customers make purchase decisions, but has data only at the aggregate level.
Let’s give examples of the sort of problem we’re trying to solve.

Example 1: Regulating a merger

First is the classic merger problem: a regulator is be interested in whether a merger will hurt customers, as might be the case if
  1. The merging firms offer similar products that appear to be in close competition, and few other options exist. And
  2. Marginal economies of scale from the merger are small.
Let’s focus on the first problem. The regulator really needs to know whether the customers of the two firms’ products perceive the products as being similar—that is, in genuine competition with one another. This is a harder task than it might appear on the surface, as many goods might look similar to an outside regulator but are really quite different. A mid-range Mercedes Benz might have similar specifications to a Toyota, but is perceived by customers as being a different product.
Ideally the regulator would have sales-level data for each customer, their purchase decisions, and second choices. But this might be impossible to get. Yet it is quite straightforward to purchase aggregate sales data at the product-market level from market research firms. So the regulator has to make do with that.

Example 2: A manager considering a new product or a new market

Managers ideally want to create goods and products where there is a strong latent demand but little competition. To do this, they need to understand the distribution of customers’ preferences over product characteristics, illustrated in the figure below by the blue contours. The manager should also understand the distribution of of competitors’ (and their own) products on those same characteristics. These are illustrated as points in the figure below. A manager might then decide to enter a market by offering a product with characteristics that customers value but where few competing products exist.
Such analysis might be quite straightforward when the manager has access to customer-transaction-level data of their competitors, but this is not feasible in most cases. Instead, they need to make do with the sorts of aggregate data available from the same research houses in the example above.

A generative model of consumer choice

These examples above have the same objective: we want to perform analysis that requires knowing about the distribution of customer preferences. And both problems face the same major constraint: we don’t observe transaction-level data for all products–we only observe aggregate sales in each market. Moreover, the most valuable aspects of consumer preferences are relative, not absolute - e.g. a manager would be interested not just in whether a particular product would sell, but how well it would sell at different price points. How much would consumers be willing to pay for a new product, and how much would that affect the prices and sales of existing products?
Even with very fine data and a few frequently purchased products, this is a difficult question to answer. It requires inferring how consumers would substitute one good for another across a range of prices for each good—the sheer number of combinations of possibilities that could be relevant is too high to even conceive of, let alone observe in data. Of course, we don’t actually have to observe every possible combination of products and prices to have a predictive model of substitution patterns that performs pretty well. We just need a tractable, flexible generative model that can smooth out the space of possibilities in a sensible way.
One extremely flexible model is the aggregate random coefficients logit model. In this model, customer i in market t has preferences over product j such that their total utility is



This says that each consumer receives utility, , from good  according to its price, , its characteristics, , some systematic “demand” or “quality” shock applicable to all customers in market , and some iid shock, . There are a couple of very important things to notice about this utility function:
  • We don’t observe  or . Because  is normally interpreted as “quality” or a “demand shock”, we assume that it is correlated with price.
  • In this specification, each customer has their own preferences over price and product characteristics. The joint distribution of  and  is the distribution of structural parameters we care about. It tells us what combinations of product characteristics and prices customers like, as well as how we expect their utility from a product to vary when the product characteristics or price change.
  • We make the assumption that  is distributed according to a Gumbel distribution. This is a convenience that allows us to use a softmax function for the probabilities of purchase. The parameterization of the Gumbel doesn’t actually matter so long as all products, customers and markets have the same location and scale parameters. Once we introduce the outside good below, we assume that the expected value of utility for the outside good is 0. This is equivalent to setting , and  to 0 for the outside good and the location of the Gumbel to  (Euler’s constant, about -.58).

Introducing an outside good

In the model, a customer purchases whichever good gives them the highest utility. This means we only care about relative utility, not absolute utility. We could add 4 or 200 to all products’ utilities, and customer choices would remain the same. This necessitates us introducing an “outside good” (with an expected utility of 0), which serves as a reference point against which all other utilities are measured. We make the assumption that it has sales equal to the potential size of the market (which we don’t know) less the sales of the goods for which we have data.

From individual utilities to individual probabilities of purchase

Under the above assumptions, it is possible to derive the probability that an individual will purchase good j. This derivation is in Train (2003)—we’ll spare you from it here. Individual purchase probabilities are just the probability that a good’s associated utility is the highest of all available good (including the outside good).


Where the denominator includes the expected value of the utility of the outside good .

Generating aggregate sales data from the model

Because this is a generative model, there is a clear mapping between our economic model (above) and the observed data. But first, what exactly is the observed data? We have the inputs, prices , and  observed product characteristics . Using these variables and the unknowns  fixed at some values, we should be able to generate the outcomes—the sales of each good in each market (some researchers use market shares instead of sales; we prefer sales as described below).
But how do the data come about? If a customer’s probability of choosing to purchase a given object is given by a combination of the product knowns and the unknowns of the model, then the market shares of each product will, for a large number of customers, be the integral of the individual probability model over the preference space. This sounds difficult, but is really quite simple: each customer has a probability of purchasing each good. If we have values for the unknowns of the model  (and the model is correct) we know the probability of customer i making each purchase. We can calculate market shares as the average of these probabilities over many customers. Note that because the probability model is non-linear, this is different to the probabilities implied by the softmax function at the average values of .
What have we done? We’ve gone from a model of individual choice making to implied market shares of heterogeneous goods. This is often where researchers’ models stop (in the famous BLP case,  is backed out to set implied market shares to actual market shares). But we want to be able to deal with measurement error—the fact that given some true market shares for each good , our data has been collected with error giving us estimates . We make the assumption that true market shares  map to observed sales (which are observed with error) through a multinomial distribution with a given market size.

Modeling price

A major difference between our model and other approaches is to model the demand shock ξjt as an explicit random variable, about which we want to perform inference. The approach we take is to model it as a latent factor, whose value is revealed partly in utilities (and market shares), and partly in prices or any other endogenous product characteristics (Petrin and Train (2010) propose a similar control function approach and show how it can substitute for IVs). This requires that we model the functional form of the true price-generating process. Here we’ll make the assumption that the natural log of prices is a linear function of product characteristics and the latent factor.

λ is known as a factor loading. We’ve used a lognormal distribution here because it places no weight on negative prices. If negative prices are possible, we should not use this formulation. We could also use a less skewed distribution if desired, such as a truncated normal. If we have instruments for price, they could be included in this model, but they are not necessary. Using this specification,  will be correlated with  in the utility model, ameliorating many of our concerns about endogenous prices.
Note that this need not be the full specification for price. BLP and its followups incorporate a structural model of price setting, in which prices are set as firms’ profit maximizers in an economic model of price competition (the model typically used is one of Nash equilibrium in Differentiated Product Bertrand Competition). Firms are assumed to observe the demand function that we are estimating and to choose prices optimally in response to their competitors’ optimal pricing decisions. Prices are therefore specified as the simultaneous solutions of the firms’ profit function first order conditions.
We could do the same here. The effect would be semi-parametric identification of the firms’ cost functions. Optimal pricing depends both on the population demand function and on firms’ costs of production. Conceptually what the structural model of pricing does is to assume that not only are consumers making purchasing decisions to maximize their utilities given posted prices, but also, the posted prices that are observed and the quantities sold are set in equilibrium - precisely where the supply curve meets the demand curve (remember that graph from econ 101?). 
If we know the shape of the demand curve, then we can use the observations of price-quantity pairs in our data to trace out the supply curve (giving us an idea of the production cost function). In classical BLP we’d be doing this ‘tracing’ procedure entirely on the basis of the theoretical model of optimal price setting. To do this within a fully generative model, we would need to account for the error in the optimal price setting (that is, since we don’t believe that firms choose their prices by literally taking first order conditions, but rather follow some qualitative/quantitative procedures that allow them to approximate the optimum in equilibrium, we need to take the approximation error into account). Employing a parametric specification of price (such as the lognormal function of observed characteristics and a latent factor as we propose here) on top of the FOC specification can do just that. Moreover, incorporating the latent factor as an explicit parameter in price-setting allows us to model the endogeneity that would otherwise result in poor estimates.

Estimating the model from aggregate market-level data

At a high level, the trick to estimating this model is to estimate the the distribution of the individual-level coefficients, rather than the actual individual-level coefficients (which we obviously cannot estimate from aggregate data). We do this by reformulating the utility model in terms of fixed and random utility, and passing in the individual random effects zi as data.
First, express utility in terms of a fixed and random portion:

 is a row vector of  independent draws from some distribution, normally a unit normal, and  is the lower triangular Cholesky factorization of , which is a  covariance matrix. To be clear,  is the covariance matrix of variations in  across customers. If the element of  corresponding to price and product characteristic 3 is negative, it means that customers who are more less sensitive to price (assuming all are negative, those whose s are closer to 0) tend to derive less utility from characteristic 3. Good estimates of  are what give us good estimates of the distribution of preferences, which is what we ultimately want.
Note that if we have information about how markets differ from one another (for instance their demographics), we could include that information in the random effects part of the model.
Given this structure, we can estimate the structural parameters (and demand shocks) using the following method:
  1. Draw a  matrix independent shocks , for some large number . We normally use the same shocks for every market.
  2. For a given draw of the structural parameters , for each market for each  calculate  and hence .
  3. Aggregate individual probabilities into predicted market shares 
  4. Model  and  as described above.
Steps 2 and 3 occur in every iteration (or, if you are using HMC, every leapfrog step) of your model estimation.

Part 2: Fake data simulation

Astute readers will be aware that we always recommend simulating fake data with known parameters for a model. Here we do precisely that. All fake data simulation is in R. The comments should describe what’s going on here.
set.seed(57)
# Dimensions of the data. 
NS <- 500 # 500 fake customers in each market
J <- 10 # 10 products
T <- 50 # 20 markets
P <- 3 # 3 covariates

# structural parameters
alpha <- -1
lambda <- .8
beta <- rnorm(P)

# Create a covariance matrix of the individual-level parameters

scales <- seq(from = .2, to = .9, length.out = P+1)

# Generate a random correlation matrix
XX <- matrix(rnorm(4*6), 6, 4)
Omega <- cor(XX)
Sigma <- diag(scales) %*% Omega %*% diag(scales)


# Product characteristics matrix
X <- matrix(rnorm(J*P), J, P)

# Easier to use if we repeat it for each market. We can have different 
# characteristics (like advertising) in each market
X_long <- do.call(rbind, replicate(T, X, simplify = F))

# structural shocks and price
xi <- rnorm(T*J, 0, 1)
xi_mat <- matrix(xi, T, J, byrow = T)
gamma <- rnorm(P)

P2 <- 3 # number of instruments
gamma2 <- rnorm(P2)
Z <- matrix(rnorm(P2*J*T), J*T, P2)

# The generative model for price
price <- exp(0 +  lambda*xi + X_long %*% gamma + rnorm(T*J, 0, 0.5))
price_mat <- matrix(price, T, J, byrow = T)

# Market size
market_size <- round(rpois(T, 30000))

# Deltas (the fixed part of utility for each product)
delta <- alpha*price + X_long %*% beta + xi
delta_mat <- matrix(delta[,1], T, J, byrow = T)

# random shocks. (alpha_shocks, beta_shocks) = z_t
z <- matrix(rnorm(NS*(P+1)), NS, P+1)

# Empty market shares. Mat is for the observed products; sales is for all goods including the outside good
shares_mat <- matrix(NA, T, J)
sales <-  matrix(NA, T, J+1)

# Loop through each matrix and generate sales for each product
for(i in 1:T) {
  
  # Latent utility matrix
  utility <- matrix(NA, NS, J)
  # Create the random component of the (alpha, beta) vector
  random_effects <-  z %*% chol(Sigma)
  
  # monte carlo integration
  for(n in 1:NS){
    utility[n,] <-t( exp(delta_mat[i,] + cbind(price_mat[i,], X) %*% random_effects[n,]))
    utility[n,] <- utility[n,]/(1 + sum(utility[n,]))
  }
  shares_mat[i,] <- colMeans(utility)
  
  # Now we're going to observe the shares with measurement error
  # Last column is the outside good
  sales[i,] <- rmultinom(1, market_size[i], c(shares_mat[i,], 1 - sum( shares_mat[i,])))
}
It should be pointed out that here  and  are correlated. This should introduce endogeneity problems in the model.