pyblp.Problem¶

class pyblp.Problem(product_formulations, product_data, agent_formulation=None, agent_data=None, integration=None, distributions=None, epsilon_scale=1.0, costs_type='linear', add_exogenous=True)

A BLP-type problem.

This class is initialized with relevant data and solved with Problem.solve().

Parameters
• product_formulations (Formulation or sequence of Formulation) –

Formulation configuration or a sequence of up to three Formulation configurations for the matrix of demand-side linear product characteristics, $$X_1$$, for the matrix of demand-side nonlinear product characteristics, $$X_2$$, and for the matrix of supply-side characteristics, $$X_3$$, respectively. If the formulation for $$X_3$$ is not specified or is None, a supply side will not be estimated. Similarly, if the formulation for $$X_2$$ is not specified or is None, the logit (or nested logit) model will be estimated.

Variable names should correspond to fields in product_data. The shares variable should not be included in the formulations for $$X_1$$ or $$X_2$$. If shares is included in the formulation for $$X_3$$, care should be taken when solving for equilibrium prices in, for example, ProblemResults.compute_prices(), since this routine assumes that marginal costs remain constant.

The prices variable should not be included in the formulation for $$X_3$$, but it should be included in the formulation for $$X_1$$ or $$X_2$$ (or both). The absorb argument of Formulation can be used to absorb fixed effects into $$X_1$$ and $$X_3$$, but not $$X_2$$. Characteristics in $$X_2$$ should generally be included in $$X_1$$. The typical exception is characteristics that are collinear with fixed effects that have been absorbed into $$X_1$$.

By default, characteristics in $$X_1$$ that do not involve prices, $$X_1^\text{ex}$$, will be combined with excluded demand-side instruments (specified below) to create the full set of demand-side instruments, $$Z_D$$. Any fixed effects absorbed into $$X_1$$ will also be absorbed into $$Z_D$$. Similarly, characteristics in $$X_3$$ that do not involve shares, $$X_3^\text{ex}$$, will be combined with the excluded supply-side instruments to create $$Z_S$$, and any fixed effects absorbed into $$X_3$$ will also be absorbed into $$Z_S$$. The add_exogenous flag can be used to disable this behavior.

Warning

Characteristics that involve prices, $$p$$, or shares, $$s$$, should always be formulated with the prices and shares variables, respectively. If another name is used, Problem will not understand that the characteristic is endogenous, so it will be erroneously included in $$Z_D$$ or $$Z_S$$, and derivatives computed with respect to prices or shares will likely be wrong. For example, to include a $$p^2$$ characteristic, include I(prices**2) in a formula instead of manually constructing and including a prices_squared variable.

• product_data (structured array-like) –

Each row corresponds to a product. Markets can have differing numbers of products. The following fields are required:

• market_ids : (object) - IDs that associate products with markets.

• shares : (numeric) - Marketshares, $$s$$, which should be between zero and one, exclusive. Outside shares should also be between zero and one. Shares in each market should sum to less than one.

• prices : (numeric) - Product prices, $$p$$.

If a formulation for $$X_3$$ is specified in product_formulations, firm IDs are also required, since they will be used to estimate the supply side of the problem:

• firm_ids : (object, optional) - IDs that associate products with firms.

Excluded instruments are typically specified with the following fields:

• demand_instruments : (numeric) - Excluded demand-side instruments, which, together with the formulated exogenous demand-side linear product characteristics, $$X_1^\text{ex}$$, constitute the full set of demand-side instruments, $$Z_D$$. To instead specify the full matrix $$Z_D$$, set add_exogenous to False.

• supply_instruments : (numeric, optional) - Excluded supply-side instruments, which, together with the formulated exogenous supply-side characteristics, $$X_3^\text{ex}$$, constitute the full set of supply-side instruments, $$Z_S$$. To instead specify the full matrix $$Z_S$$, set add_exogenous to False.

The recommendation in Conlon and Gortmaker (2020) is to start with differentiation instruments of Gandhi and Houde (2017), which can be built with build_differentiation_instruments(), and then compute feasible optimal instruments with ProblemResults.compute_optimal_instruments() in the second stage.

For guidance on how to construct instruments and add them to product data, refer to the examples in the documentation for the build_blp_instruments() and build_differentiation_instruments() functions.

If firm_ids are specified, custom ownership matrices can be specified as well:

• ownership : (numeric, optional) - Custom stacked $$J_t \times J_t$$ ownership or product holding matrices, $$\mathscr{H}$$, for each market $$t$$, which can be built with build_ownership(). By default, standard ownership matrices are built only when they are needed to reduce memory usage. If specified, there should be as many columns as there are products in the market with the most products. Rightmost columns in markets with fewer products will be ignored.

Note

Fields that can have multiple columns (demand_instruments, supply_instruments, and ownership) can either be matrices or can be broken up into multiple one-dimensional fields with column index suffixes that start at zero. For example, if there are three columns of excluded demand-side instruments, a demand_instruments field with three columns can be replaced by three one-dimensional fields: demand_instruments0, demand_instruments1, and demand_instruments2.

To estimate a nested logit or random coefficients nested logit (RCNL) model, nesting groups must be specified:

• nesting_ids (object, optional) - IDs that associate products with nesting groups. When these IDs are specified, rho must be specified in Problem.solve() as well.

To use certain types of micro moments, product IDs must be specified:

Finally, clustering groups can be specified to account for within-group correlation while updating the weighting matrix and estimating standard errors:

• clustering_ids (object, optional) - Cluster group IDs, which will be used if W_type or se_type in Problem.solve() is 'clustered'.

Along with market_ids, firm_ids, nesting_ids, product_ids, clustering_ids, and prices, the names of any additional fields can typically be used as variables in product_formulations. However, there are a few variable names such as 'X1', which are reserved for use by Products.

• agent_formulation (Formulation, optional) – Formulation configuration for the matrix of observed agent characteristics called demographics, $$d$$, which will only be included in the model if this formulation is specified. Since demographics are only used if there are demand-side nonlinear product characteristics, this formulation should only be specified if $$X_2$$ is formulated in product_formulations. Variable names should correspond to fields in agent_data.

• agent_data (structured array-like, optional) –

Each row corresponds to an agent. Markets can have differing numbers of agents. Since simulated agents are only used if there are demand-side nonlinear product characteristics, agent data should only be specified if $$X_2$$ is formulated in product_formulations. If agent data are specified, market IDs are required:

• market_ids : (object) - IDs that associate agents with markets. The set of distinct IDs should be the same as the set in product_data. If integration is specified, there must be at least as many rows in each market as the number of nodes and weights that are built for the market.

If integration is not specified, the following fields are required:

• weights : (numeric, optional) - Integration weights, $$w$$, for integration over agent choice probabilities.

• nodes : (numeric, optional) - Unobserved agent characteristics called integration nodes, $$\nu$$. If there are more than $$K_2$$ columns (the number of demand-side nonlinear product characteristics), only the first $$K_2$$ will be retained. If any columns of sigma in Problem.solve() are fixed at zero, only the first few columns of these nodes will be used.

The convenience function build_integration() can be useful when constructing custom nodes and weights.

Note

If nodes has multiple columns, it can be specified as a matrix or broken up into multiple one-dimensional fields with column index suffixes that start at zero. For example, if there are three columns of nodes, a nodes field with three columns can be replaced by three one-dimensional fields: nodes0, nodes1, and nodes2.

Along with market_ids, the names of any additional fields can be typically be used as variables in agent_formulation. The exception is the name 'demographics', which is reserved for use by Agents.

• integration (Integration, optional) –

Integration configuration for how to build nodes and weights for integration over agent choice probabilities, which will replace any nodes and weights fields in agent_data. This configuration is required if nodes and weights in agent_data are not specified. It should not be specified if $$X_2$$ is not formulated in product_formulations.

If this configuration is specified, $$K_2$$ columns of nodes (the number of demand-side nonlinear product characteristics) will be built. However, if sigma in Problem.solve() is left unspecified or specified with columns fixed at zero, fewer columns will be used.

• distributions (sequence of str, optional) –

Random coefficient distributions. By default, random coefficients in (3) are assumed to be normally distributed. Non-default distributions can be specified with a list of the following supported strings:

• 'normal' (default) - The random coefficient is assumed to be normal.

• 'lognormal' - The random coefficient is assumed to be lognormal. The coefficient’s column in (3) is exponentiated before being pre-multiplied by $$X_2$$.

The list should have as many strings as there are columns in $$X_2$$. Each string determines the distribution of the random coefficient on the corresponding product characteristic in $$X_2$$.

A typical example of a lognormal coefficient is one on prices. Implementing this typically involves having a I(-prices) in the formulation for $$X_2$$, and instead of including prices in $$X_1$$, including a 1 in the agent_formulation. Then the corresponding coefficient in $$\Pi$$ will serve as the mean parameter for the lognormal random coefficient on negative prices, $$-p_{jt}$$.

• epsilon_scale (float, optional) –

Factor by which the Type I Extreme Value idiosyncratic preference term, $$\epsilon_{ijt}$$, is scaled. By default, $$\epsilon_{ijt}$$ is not scaled. The typical use of this parameter is to approximate the pure characteristics model of Berry and Pakes (2007) by choosing a value smaller than 1.0. As this scaling factor approaches zero, the model approaches the pure characteristics model in which there is no idiosyncratic preference term.

In practice, this is implemented by dividing $$V_{ijt} = \delta_{jt} + \mu_{ijt}$$ by the scaling factor when solving for the mean utility $$\delta_{jt}$$. For small scaling factors, this leads to large values of $$V_{ijt}$$, which when exponentiated in the logit expression can lead to overflow issues discussed in Berry and Pakes (2007). The safe versions of the contraction mapping discussed in the documentation for fp_type in Problem.solve() (which is used by default) eliminate overflow issues at the cost of introducing fewer (but still common for a small scaling factor) underflow issues. Throughout the contraction mapping, some values of the simulated shares $$s_{jt}(\delta, \theta)$$ can underflow to zero, causing the contraction to fail when taking logs. By default, shares_bounds in Problem.solve() bounds these simulated shares from below by 1e-300, which eliminates these underflow issues at the cost of making it more difficult for iteration routines to converge.

With this in mind, scaling epsilon is not supported for nonlinear contractions, and is also not supported when there are nesting groups, since these further complicate the problem. In practice, if the goal is to approximate the pure characteristics model, it is a good idea to slowly decrease the scale of epsilon (e.g., starting with 0.5, trying 0.1, etc.) until the contraction begins to fail. To further decrease the scale, there are a few things that can help. One is passing a different Iteration configuration to iteration in Problem.solve(), such as 'lm', which can be robust in this situation. Another is to set pyblp.options.dtype = np.longdouble when on a system that supports extended precision (see options for more information about this) and choose a smaller lower bound by configuring shares_bounds in Problem.solve(). Ultimately the model will stop being solvable at a certain point, and this point will vary by problem, so approximating the pure characteristics model requires some degree of experimentation.

• costs_type (str, optional) –

Functional form of the marginal cost function $$\tilde{c} = f(c)$$ in (9). The following specifications are supported:

• 'linear' (default) - Linear specification: $$\tilde{c} = c$$.

• 'log' - Log-linear specification: $$\tilde{c} = \log c$$.

This specification is only relevant if $$X_3$$ is formulated.

Whether to add characteristics in $$X_1$$ that do not involve prices, $$X_1^\text{ex}$$, to the demand_instruments field in product_data (including absorbed fixed effects), and similarly, whether to add characteristics in $$X_3$$ that do not involve shares, $$X_3^\text{ex}$$, to the supply_instruments field. This is by default True so that only excluded instruments need to be specified.

If this is set to False, demand_instruments and supply_instruments should specify the full sets of demand- and supply-side instruments, $$Z_D$$ and $$Z_S$$, and fixed effects should be manually absorbed (for example, with the build_matrix() function). This behavior can be useful, for example, when price is not the only endogenous product characteristic over which consumers have preferences. This model could be correctly estimated by manually adding the truly exogenous characteristics in $$X_1$$ to $$Z_D$$.

Warning

If this flag is set to False because there are multiple endogenous product characteristics, care should be taken when including a supply side or computing optimal instruments. These routines assume that price is the only endogenous variable over which consumers have preferences.

product_formulations

Formulation configurations for $$X_1$$, $$X_2$$, and $$X_3$$, respectively.

Type

Formulation or sequence of Formulation

agent_formulation

Formulation configuration for $$d$$.

Type

Formulation

products

Product data structured as Products, which consists of data taken from product_data along with matrices built according to Problem.product_formulations. The data_to_dict() function can be used to convert this into a more usable data type.

Type

Products

agents

Agent data structured as Agents, which consists of data taken from agent_data or built by integration along with any demographics built according to Problem.agent_formulation. The data_to_dict() function can be used to convert this into a more usable data type.

Type

Agents

unique_market_ids

Unique market IDs in product and agent data.

Type

ndarray

unique_firm_ids

Unique firm IDs in product data.

Type

ndarray

unique_product_ids

Unique product IDs in product data.

Type

ndarray

unique_nesting_ids

Unique nesting group IDs in product data.

Type

ndarray

distributions

Random coefficient distributions.

Type

list of str

epsilon_scale

Factor by which the Type I Extreme Value idiosyncratic preference term, $$\epsilon_{ijt}$$, is scaled.

Type

float

costs_type

Functional form of the marginal cost function $$\tilde{c} = f(c)$$.

Type

str

T

Number of markets, $$T$$.

Type

int

N

Number of products across all markets, $$N$$.

Type

int

F

Number of firms across all markets, $$F$$.

Type

int

I

Number of agents across all markets, $$I$$.

Type

int

K1

Number of demand-side linear product characteristics, $$K_1$$.

Type

int

K2

Number of demand-side nonlinear product characteristics, $$K_2$$.

Type

int

K3

Number of supply-side product characteristics, $$K_3$$.

Type

int

D

Number of demographic variables, $$D$$.

Type

int

MD

Number of demand-side instruments, $$M_D$$, which is typically the number of excluded demand-side instruments plus the number of exogenous demand-side linear product characteristics, $$K_1^\text{ex}$$.

Type

int

MS

Number of supply-side instruments, $$M_S$$, which is typically the number of excluded supply-side instruments plus the number of exogenous supply-side linear product characteristics, $$K_3^\text{ex}$$.

Type

int

ED

Number of absorbed dimensions of demand-side fixed effects, $$E_D$$.

Type

int

ES

Number of absorbed dimensions of supply-side fixed effects, $$E_S$$.

Type

int

H

Number of nesting groups, $$H$$.

Type

int

Examples

Methods

 solve([sigma, pi, rho, beta, gamma, …]) Solve the problem.