brycewang-stanford/structural-modeling

Structural Modeling

Reference for implementing structural econometric models: from economic model to moment conditions to estimated parameters. Covers the full workflow of taking a theoretical model, deriving its empirical content, and recovering structural parameters from data.

When to Use This Skill

Use when the user is:

• Specifying a structural model and deriving moment conditions
• Implementing NFXP or MPEC estimation routines
• Working with BLP-style demand systems (random coefficients logit)
• Building dynamic discrete choice models (Rust, Hotz-Miller CCP)
• Estimating auction models (first-price, ascending, common value)
• Debugging convergence failures in structural estimation
• Choosing between estimation approaches for a given model

Skip when:

• The task is reduced-form causal inference (use causal-inference skill)
• The task is pure simulation design (use numerical-auditor agent)
• The user just needs standard regression (statsmodels/linearmodels suffice)

Quick Reference: Structural Methods

| Method | Use Case | Key Package | Estimator | |--------|----------|-------------|-----------| | NFXP | Dynamic discrete choice (small state space) | scipy.optimize | MLE / GMM | | MPEC | Dynamic discrete choice (large state space, slow inner loop) | cyipopt (IPOPT) | MLE / GMM | | BLP | Differentiated products demand with RC logit | pyblp | GMM (2-step) | | CCP (Hotz-Miller) | Dynamic models, counterfactuals not needed | scipy | 2-step semiparametric | | GPV | First-price auctions, nonparametric values | scipy | Nonparametric | | Ascending auction | English auctions, private values | scipy | MLE on order statistics |

The Structural Estimation Workflow

Every structural estimation follows the same logical arc:

Economic Model → Equilibrium/Decision Rule → Observable Implications
    → Moment Conditions → Estimator → Optimization → Inference

Step 1: Model Specification

Define primitives clearly before writing any code:

# model_spec.py — Document structural primitives
"""
Model: Single-agent optimal stopping (Rust 1987 bus engine replacement)

State:    x_t ∈ {0, 1, ..., X_max}  (mileage bin)
Action:   a_t ∈ {0, 1}  (0 = maintain, 1 = replace)
Flow payoff:
    u(x, 0; θ) = -θ_1 * x - θ_2 * x²     (maintenance cost)
    u(x, 1; θ) = -RC                        (replacement cost)
Discount:  β = 0.9999 (fixed)
Shocks:    ε ~ Type 1 Extreme Value (logit errors)
"""

Document these before writing estimation code: agents, information, timing, payoff functional form, equilibrium concept.

Step 2: Derive Moment Conditions

| Source | Example | Estimator | |--------|---------|-----------| | Optimality conditions (FOCs) | Euler equations, Bellman optimality | GMM | | Equilibrium restrictions | Market clearing, Nash conditions | GMM / ML | | Distributional assumptions | Choice probabilities under logit errors | MLE | | Exclusion restrictions | Cost shifters excluded from demand | IV-GMM |

Key question: Just-identified → method of moments; over-identified → GMM with optimal weighting matrix; under-identified → revisit assumptions.

NFXP vs MPEC

Two dominant paradigms for models with latent quantities (unobserved heterogeneity, future expectations, equilibrium objects):

NFXP (Nested Fixed-Point): Solve the model in an inner loop for each parameter guess, evaluate likelihood/moments in an outer loop. Conceptually simple; inner loop must fully converge at every iteration — requires tight tolerance (1e-12, not 1e-6; see Su & Judd 2012).

MPEC (Mathematical Programming with Equilibrium Constraints): Reformulate as a single constrained optimization. No inner loop — solver handles everything; can be faster for large state spaces; requires IPOPT or KNITRO.

| Factor | Favors NFXP | Favors MPEC | |--------|-------------|-------------| | State space | Small (< 500 states) | Large (> 1000 states) | | Inner loop | Fast convergence (rate < 0.9) | Slow or fragile | | Solver availability | scipy.optimize sufficient | IPOPT/KNITRO available | | Debugging | Easier — isolate inner vs outer | Harder to diagnose constraint violations |

For full NFXP and MPEC code (Rust 1987 bus engine model), see references/estimation-methods.md.

BLP Demand Estimation

BLP (Berry, Levinsohn, Pakes 1995) is the workhorse for differentiated products demand. Use PyBLP whenever possible — it handles the difficult numerical details correctly.

import pyblp

# Define the problem
problem = pyblp.Problem(
    product_formulations=(
        pyblp.Formulation('1 + prices + x1 + x2'),       # linear (β)
        pyblp.Formulation('1 + prices + x1'),              # random coefficients (Σ)
    ),
    product_data=product_data,
    agent_data=agent_data
)

# Solve — always use multiple starting values; BLP objective is non-convex
results = problem.solve(
    sigma=sigma_init,
    optimization=pyblp.Optimization('l-bfgs-b', {'gtol': 1e-8}),
    iteration=pyblp.Iteration('squarem', {'atol': 1e-14}),
    method='2s'
)

For the full multi-start loop, two-step GMM, elasticity checks, instrument selection, and marginal cost computation, see references/estimation-methods.md.

BLP Diagnostics Checklist:

• [ ] First-stage F > 10 for price instruments
• [ ] Run 10+ random starts (objective is non-convex)
• [ ] Own-price elasticities all negative: results.compute_elasticities('prices')
• [ ] All markets converged: results.fp_converged.all()
• [ ] Marginal costs positive: results.compute_costs()
• [ ] Inner loop atol <= 1e-14 (tighter is safer)

Dynamic Discrete Choice

Rust (1987) NFXP: Full solution — solve the Bellman equation by value function iteration at every outer iteration. Use for models where counterfactuals require the full model.

Hotz-Miller CCP: Two-step semiparametric approach. Step 1: estimate conditional choice probabilities nonparametrically. Step 2: form pseudo-value functions for a linear regression. Faster; less efficient; sufficient when counterfactuals are not needed.

| Feature | Full Solution (NFXP/MPEC) | CCP (Hotz-Miller) | |---------|---------------------------|---------------------| | Computational cost | High (solve DP at each θ) | Low (no DP solving) | | Efficiency | Efficient (MLE) | Less efficient (2-step) | | Counterfactuals | Natural (full model available) | Must resolve for new policies |

For full CCP implementation code, see references/estimation-methods.md.

Auction Models

First-price sealed-bid (GPV): Guerre, Perrigne, Vuong (2000) — invert the bidding equilibrium condition v(b) = b + G(b)/((n-1)g(b)) to recover latent values nonparametrically from observed bids.

Ascending (English): In IPV setting, transaction price = second-highest value. Use MLE on order statistics to recover the value distribution.

Common value: Requires accounting for winner's curse. Li-Perrigne-Vuong (2002) approach; typically requires parametric assumptions.

For full GPV estimator code, ascending auction MLE, and validation diagnostics, see references/estimation-methods.md.

Method Selection

When to use structural vs. reduced-form:

• Structural: Need to evaluate counterfactual policies, recover preference parameters, or model strategic interactions
• Reduced-form: Need a credible causal estimate of a specific treatment effect with minimal assumptions

Within structural:

1. Static discrete choice with heterogeneity? → BLP / mixed logit
2. Dynamic single-agent optimal stopping? → NFXP (small state) or MPEC (large state)
3. Counterfactuals not needed, data rich? → CCP estimator
4. Auction data? → GPV (first-price) or order statistics MLE (ascending)
5. Market-level entry/exit? → Bresnahan-Reiss or Ciliberto-Tamer (see game-theory skill)

Common Anti-Patterns

| Anti-Pattern | Problem | Better Approach | |--------------|---------|-----------------| | Estimating β jointly with payoff parameters | Notoriously poorly identified; flat objective | Fix β at reasonable value (0.95, 0.99) or calibrate externally | | Loose inner loop tolerance (1e-6) | Optimizer sees noise; spurious convergence | Use 1e-12 or tighter; see Su & Judd (2012) | | Single starting value | Structural objectives are non-convex | Use 10+ random starts plus grid search | | Ignoring simulation error in simulated MLE/MSM | Biased standard errors | Use enough draws (R >> N) or bias-correct | | Numerical gradients with default step size | Inaccurate for poorly scaled problems | Use central differences or analytic gradients (JAX) | | Hard-coding state space discretization | Results sensitive to grid coarseness | Test sensitivity to grid refinement |

JAX Acceleration

For GPU-accelerated structural estimation (JIT compilation, autodiff, vmap for simulated moments, differentiable fixed-point iteration with lax.while_loop), see references/jax-guide.md.

Integration with compound-science

• numerical-auditor — Systematic convergence review: gradient norms, conditioning, tolerance sensitivity
• numerical-auditor — DGP formalization, Monte Carlo studies, convergence review
• identification-critic — Verify equilibrium existence, uniqueness, stability, comparative statics
• econometric-reviewer — Reviews moment-matching strategy, parameter identification, sensitivity to targets
• /estimate — Full estimation pipeline with quality gates

Additional References

• references/estimation-methods.md — Full code: BLP multi-start, NFXP Bellman solver, MPEC cyipopt formulation, Hotz-Miller CCP, GPV auction estimator
• references/diagnostics-and-se.md — Convergence failure diagnosis, numerical safeguards (logsumexp, conditioning), GMM sandwich SEs, parametric bootstrap
• references/jax-guide.md — JAX JIT/autodiff for structural objectives, vmap for simulation, lax.while_loop for differentiable contraction mappings