An example of market-clearing assessment
In diseq: Estimation Methods for Markets in Equilibrium and Disequilibrium

Package diseq is deprecated. Please use package markets instead.

This short tutorial gives an example of statistically assessing whether a market is in an equilibrium state. The tutorial assumes some familiarity with the concepts and the functionality of the package. The basic_usage vignette can be helpful in acquiring this familiarity.

if (requireNamespace("knitr", quietly = TRUE)) {
  knitr::opts_chunk$set(collapse = TRUE, comment = "#>")
}

Setup the environment

Load the required libraries.

library(diseq)
library(magrittr)

Prepare the data. Here, we simply simulate data using a data generating process for a market in equilibrium.

nobs <- 1000
tobs <- 5

alpha_d <- -3.9
beta_d0 <- 28.9
beta_d <- c(2.1, -0.7)
eta_d <- c(3.5, 6.25)

alpha_s <- 2.8
beta_s0 <- 26.2
beta_s <- c(2.65)
eta_s <- c(1.15, 4.2)

sigma_d <- 0.8
sigma_s <- 1.1
rho_ds <- 0.0

seed <- 42

eq_data <- simulate_data(
  "equilibrium_model", nobs, tobs,
  alpha_d, beta_d0, beta_d, eta_d,
  alpha_s, beta_s0, beta_s, eta_s,
  NA, NA, c(NA),
  sigma_d = sigma_d, sigma_s = sigma_s, rho_ds = rho_ds,
  seed = seed
)

Estimate the models

Prepare the basic parameters for model initialization.

verbose <- 2
correlated_shocks <- TRUE
formula <-   Q | P | id | date ~ P + Xd1 + Xd2 + X1 + X2 | P + Xs1 + X1 + X2

Set the estimation parameters.

optimization_method <- "BFGS"
optimization_options <- list(maxit = 10000, reltol = 1e-8)

Using the above parameterization, construct and estimate the model objects. Here we estimate two equilibrium models and four disequilibrium models. All the models are constructed using the simulated data from a model of market in equilibrium.

eq_reg <- equilibrium_model(
  formula, eq_data[eq_data$date != 1, ],
  correlated_shocks = correlated_shocks, verbose = verbose,
  estimation_options = list(method = "2SLS")
)
eq_fit <- equilibrium_model(
  formula, eq_data[eq_data$date != 1, ],
  correlated_shocks = correlated_shocks, verbose = verbose,
  estimation_options = list(
    control = optimization_options, method = optimization_method
  )
)
bs_fit <- diseq_basic(
  formula, eq_data[eq_data$date != 1, ],
  correlated_shocks = correlated_shocks, verbose = verbose,
  estimation_options = list(
    control = optimization_options, method = optimization_method
  )
)
da_fit <- diseq_deterministic_adjustment(
  formula, eq_data,
  correlated_shocks = correlated_shocks, verbose = verbose,
  estimation_options = list(
    control = optimization_options, method = optimization_method
  )
)

Post estimation analysis

Summaries

All the models provide estimates for the simulated data. Even with simulated data, it is difficult to assess which model performs better by examining only the summaries in separation or collectively.

summary(eq_reg@fit[[1]]$first_stage_model)
summary(eq_reg)
summary(eq_fit)
summary(bs_fit)
summary(da_fit)

Model selection

The deterministic adjustment model has price dynamics that are analogous to excess demand and estimates one extra parameter. The directional model estimates one parameter less as the model does not have enough equations to identify prices in both demand and supply equations. The estimated parameters are summarized as follows.

da_coef <- coef(da_fit)
coef_names <- names(da_coef)

sim_coef <- c(
  alpha_d, beta_d0, beta_d, eta_d,
  alpha_s, beta_s0, beta_s, eta_s,
  NA,
  sigma_d, sigma_s,
  rho_ds
)

coef_tbl <- function(fit) {
    tibble::tibble(coef = names(coef(fit)),!!substitute(fit) := coef(fit))
}

comp <- coef_tbl(da_fit) %>% 
    dplyr::left_join(coef_tbl(bs_fit), by = "coef") %>%
    dplyr::left_join(coef_tbl(eq_reg), by = "coef") %>%
    dplyr::left_join(coef_tbl(eq_fit), by = "coef") %>%
    dplyr::mutate(sim = sim_coef) %>%
    dplyr::mutate(sim = sim_coef,
                  da_fit_err = abs(da_fit - sim),
                  bs_fit_err = abs(bs_fit - sim),
                  eq_fit_err = abs(eq_fit - sim)) 

comp

Since we have used simulated data, we can calculate the average absolute error of the parameter estimation for each of the models. The population values are unknown in practice, and this calculation is impossible.

model_errors <- colMeans(comp[, grep("err", colnames(comp))], na.rm = TRUE) 
model_errors

Moreover, the average absolute error cannot provide an overall estimation assessment as the market models have different parameter spaces. To assess the overall model performance, one can instead use an information criterion.

fits <- c(da_fit, bs_fit, eq_fit)
model_names <- sapply(fits, function(m) m@model_type_string)
model_obs <- sapply(fits, nobs)
aic <- sapply(fits, AIC)
df <- sapply(fits, function(m) attr(logLik(m), "df"))
seltbl <- tibble::tibble(Model = model_names, AIC = aic,
                         D.F = df, Obs. = model_obs,
                         Abs.Error = model_errors) %>%
  dplyr::arrange(AIC)
seltbl