Python Economic Numerical Computing

• Author：Wenli Xu
• Email： wlxu@cityu.edu.mo
• 2026-03-11

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Overview

Best practices for macroeconomic modeling (DSGE/HANK), causal inference, and data analysis in Python. Core principle: vectorize first, accelerate loops with Numba, keep code structure aligned with economic theory.

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Library Quick Reference

Use Case	Preferred Libraries
Numerical core	numpy, scipy
Loop acceleration	numba (@njit, @njit(parallel=True))
Economics toolkit	quantecon
HANK / sequence space	sequence_jacobian (SSJ)
Heterogeneous agents	HARK
Linear models with FE	pyfixest (pip install pyfixest)
DID / DD / DDD	diff-diff (pip install diff-diff)
IV / 2SLS / GMM	linearmodels (or pyfixest for panel IV with FE)
RD / RDD / RKD	rdrobust, rddensity, rdlocrand
Synthetic Control	pysynth, synth_control, sdid
Matching	causalml, pymatch, econml
Causal ML / DML	econml, dowhy
Data manipulation	pandas, polars (large datasets)
Visualization	matplotlib, seaborn

---

DSGE Models

Linearization and Solution (Blanchard-Kahn)

import numpy as np
from scipy.linalg import ordqz

def solve_bk(A, B, n_fwd):
    """
    Solve linear DSGE: A E_t[x_{t+1}] = B x_t + C eps_t
    n_fwd: number of forward-looking variables
    Returns decision rule matrix P such that x_t = P x_{t-1} + ...
    """
    AA, BB, alpha, beta, Q, Z = ordqz(A, B, sort='ouc')
    n = A.shape[0]
    Z21 = Z[n - n_fwd:, :n - n_fwd]
    Z22 = Z[n - n_fwd:, n - n_fwd:]
    P = -np.linalg.solve(Z22, Z21)
    return P

Perturbation Methods (Second-Order Approximation)

• Use quantecon.lqcontrol for LQ problems
• Higher-order perturbation: perturbpy or manual implementation
• Steady-state solving: scipy.optimize.fsolve / root

---

HANK Models

Sequence-Space Jacobian Method (SSJ)

import sequence_jacobian as sj

# 1. Define steady-state blocks
@sj.simple
def household_ss(r, w, beta, sigma):
    # Return steady-state aggregates
    ...

# 2. Build DAG
model = sj.create_model([household_block, firm_block, market_clearing],
                         name='HANK')

# 3. Solve steady state
ss = model.solve_steady_state(calibration, unknowns, targets)

# 4. Compute Jacobians → solve transition dynamics
G = model.solve_jacobian(ss, unknowns, targets, T=300)

Value Function Iteration — Numba Accelerated

from numba import njit
import numpy as np

@njit
def vfi(V0, a_grid, y_grid, r, beta, sigma, tol=1e-8, max_iter=1000):
    """Heterogeneous agent VFI over asset grid × income grid"""
    n_a, n_y = len(a_grid), len(y_grid)
    V = V0.copy()
    policy = np.zeros((n_a, n_y))

    for it in range(max_iter):
        V_new = np.empty_like(V)
        for ia in range(n_a):
            for iy in range(n_y):
                best_val = -1e10
                best_a = 0
                for ia2 in range(n_a):
                    c = (1 + r) * a_grid[ia] + y_grid[iy] - a_grid[ia2]
                    if c <= 0:
                        continue
                    u = c ** (1 - sigma) / (1 - sigma)
                    val = u + beta * V[:, iy].mean()  # use transition matrix in practice
                    if val > best_val:
                        best_val = val
                        best_a = ia2
                V_new[ia, iy] = best_val
                policy[ia, iy] = a_grid[best_a]
        if np.max(np.abs(V_new - V)) < tol:
            break
        V = V_new
    return V, policy

Distribution Iteration (Young 2010)

def iterate_distribution(policy_idx, trans_mat, dist0, T=500):
    """Iterate joint distribution to steady state given policy indices and income transition matrix"""
    dist = dist0.copy()
    n_a, n_y = dist.shape
    for _ in range(T):
        dist_new = np.zeros_like(dist)
        for iy in range(n_y):
            for iy2 in range(n_y):
                dist_new[policy_idx[:, iy], iy2] += dist[:, iy] * trans_mat[iy, iy2]
        dist = dist_new
    return dist

---

Linear Models with Fixed Effects (pyfixest)

Rule: For any OLS/Poisson/Logit with fixed effects, use pyfixest. It mirrors R's fixest syntax.

import pyfixest as pf

# OLS with unit + time FE, cluster-robust SEs
fit = pf.feols("y ~ treat_post | unit + year",
               data=df, vcov={"CRV1": "id"})
fit.summary()

# Multiple high-dimensional FE (Frisch-Waugh absorbed)
fit = pf.feols("y ~ x1 + x2 | unit + year + industry",
               data=df, vcov={"CRV1": "id"})

# Wild cluster bootstrap (few clusters, <50)
fit = pf.feols("y ~ treat_post | unit + year",
               data=df, vcov={"CRV1": "id"})
fit.wildboottest(param="treat_post", B=9999, seed=42)

# Event study via i() syntax
fit = pf.feols("y ~ i(rel_year, ref=-1) | unit + year",
               data=df, vcov={"CRV1": "id"})
pf.iplot(fit)  # event study plot

# Poisson (count / log-linear) with FE
fit_pois = pf.fepois("y ~ treat_post | unit + year",
                     data=df, vcov={"CRV1": "id"})

# Access results
fit.coef()           # coefficient estimates
fit.se()             # standard errors
fit.pvalue()         # p-values
fit.confint()        # confidence intervals
fit._N               # number of observations

pyfixest vs statsmodels

Use case	Use
OLS / WLS with any FE	pyfixest
Poisson / logit with FE	pyfixest
Wild bootstrap	pyfixest
Time-series ARIMA, VAR	statsmodels
MLE / GLM without FE	statsmodels

---

Causal Inference: DID / DD / DDD Methods

Rule: For any DiD, DD, DDD, or staggered difference-in-differences design, use diff-diff and follow the General Empirical Workflow below.

Source: https://github.com/igerber/diff-diff | https://github.com/wenddymacro/A-General-Empirical-Workflow-for-DID

diff-diff Estimator Reference

Alias	Class	Use When
DiD	DifferenceInDifferences	Basic 2×2 DiD
TWFE	TwoWayFixedEffects	Standard panel DiD
EventStudy	MultiPeriodDiD	Dynamic effects / event study
CS	CallawaySantAnna	Staggered adoption, heterogeneous effects
SA	SunAbraham	Staggered, avoids negative weights
BJS	ImputationDiD	Borusyak et al. imputation approach
SDiD	SyntheticDiD	Synthetic DiD
DDD	TripleDifference	Triple difference

Key API Parameters

from diff_diff import DiD, TWFE, EventStudy, CS, SA, BJS, DDD

# Common fit() arguments
results = estimator.fit(
    data,
    outcome='y',           # dependent variable
    treatment='treated',   # binary treatment indicator
    time='post',           # binary post-period (or period var for panel)
    unit='id',             # unit identifier (panel)
    covariates=['x1','x2'],# control variables
    absorb=['region'],     # high-dim fixed effects (within-transform)
    cluster='id',          # clustered standard errors
    robust=True,           # HC1 robust SEs
    inference='wild_bootstrap',  # for few clusters (<50)
    n_bootstrap=999,
)

# Results
results.att           # ATT estimate
results.se            # standard error
results.p_value
results.conf_int      # confidence interval tuple
results.print_summary()
results.to_dataframe()

DDD (Triple Difference)

from diff_diff import DDD

ddd = DDD()
results = ddd.fit(
    data,
    outcome='y',
    treatment='treated',
    time='post',
    third_diff='group_var',  # third differencing dimension
    cluster='id',
)

---

DID Empirical Workflow (11 Steps)

Follow this workflow for every DID/DD/DDD paper or analysis.

Step 1 — Data & Descriptive Statistics

• Construct panel: define units, time span, treatment variable
• Document data sources, missing values, winsorization
• Describe cohort structure (treated units per cohort, policy onset dates)
• Table: full-sample stats + treated vs. control comparison with t-tests
• Pre-treatment covariate balance test

Covariate types:

Type	Form	Purpose
Covariates	Pre-treatment, time-invariant	Condition parallel trends
Control variables	Baseline × time trend	Absorb residual heterogeneity

Step 2 — Identification Strategy

State and justify:

1. Parallel trends — partially testable via pre-period event study
2. SUTVA / no interference — argue no cross-unit spillovers
3. No anticipation — pre-period coefficients should be zero

Document policy assignment mechanism; cite policy documents for exogeneity.

Step 3 — Baseline Regression

Model: $$Y_{it} = \alpha + \beta(\text{Treat}i \times \text{Post}{it}) + \gamma W_i + \delta(Z_i^{pre} \times t) + \mu_i + \lambda_t + \varepsilon_{it}$$

Run six progressive specifications (M1–M6):

Model	Unit FE	Time FE	Covariates	Baseline×Trend	Regional FE	Unit Trend
M1	✓	—	—	—	—	—
M2	✓	✓	—	—	—	—
M3	✓	✓	✓	✓	—	—
M4	✓	✓	✓	✓	✓	—
M5	✓	✓	✓	✓	✓	✓
M6	✓	✓	✓	✓	Industry×Year	✓

Coefficient stability across M1→M6 supports identification. Report Oster (2019) δ* (selection bias ratio); |δ*| > 1 = basic robustness, |δ*| > 2 = strong.

import pyfixest as pf

# M1: unit FE only
pf.feols("y ~ treat_post | unit", data=df, vcov={"CRV1": "id"}).summary()

# M2: unit + time FE
pf.feols("y ~ treat_post | unit + year", data=df, vcov={"CRV1": "id"}).summary()

# M3–M4: add covariates and baseline×trend
pf.feols("y ~ treat_post + x1 + x2 + baseline:year | unit + year",
         data=df, vcov={"CRV1": "id"}).summary()

# M5: + regional FE
pf.feols("y ~ treat_post + x1 + x2 + baseline:year | unit + year + region",
         data=df, vcov={"CRV1": "id"}).summary()

# M6: + industry×year FE
pf.feols("y ~ treat_post + x1 + x2 + baseline:year | unit + industry^year",
         data=df, vcov={"CRV1": "id"}).summary()

Step 4 — Parallel Trends: Event Study Plots

from diff_diff import EventStudy

es = EventStudy()
res = es.fit(data, outcome='y', treatment='treated',
             unit='id', time='year', base_period=-1,
             cluster='id')
res.plot()  # shows pre/post coefficients with CIs

Plot standards:

• Y-axis: "Coefficient (ATT)"; X-axis: "Years relative to policy"
• Reference lines at 0; base period k = −1 omitted
• Show M6 in main text; M1–M5 in appendix
• Pre-period coefficients (k ∈ {−4,−3,−2}) should be statistically insignificant

Run all estimators for staggered designs:

from diff_diff import CS, SA, BJS

# Callaway & Sant'Anna (2021)
cs = CS().fit(data, outcome='y', unit='id', time='year',
              cohort='treat_year', control_group='never_treated', cluster='id')

# Sun & Abraham (2021)
sa = SA().fit(data, outcome='y', unit='id', time='year',
              cohort='treat_year', cluster='id')

# Borusyak et al. (2024) imputation
bjs = BJS().fit(data, outcome='y', unit='id', time='year',
                cohort='treat_year', horizons=range(5), cluster='id')

Step 5 — Parallel Trends Sensitivity: HonestDiD

Use Rambachan-Roth (2023) bounds to quantify robustness to parallel trends violations.

# After event study, extract pre/post coefficients and covariance
# Pass to HonestDiD (R package via rpy2, or use diff-diff's built-in honest DiD)
from diff_diff import EventStudy

es = EventStudy()
res = es.fit(data, ..., honest_did=True,
             sensitivity_constraint='smoothness')
res.plot_honest_did()  # shows identified set under relaxed PT assumption

Step 6 — Rule Out Alternative Explanations

Threat	Test
Spatial spillovers	Geographic placebo; effect by distance from treated units
Anticipation effects	Pre-period event-study coefficients ≈ 0
Policy overlap	Exclude or control for concurrent policies

Step 7 — Robustness Checks

from diff_diff import DiD
from diff_diff.diagnostics import PlaceboTest, GoodmanBaconDecomposition

# Goodman-Bacon decomposition (TWFE bias diagnosis)
gb = GoodmanBaconDecomposition().fit(data, outcome='y', treatment='treated',
                                      unit='id', time='year')
gb.plot()

# Placebo tests
placebo = PlaceboTest(method='fake_timing').fit(data, ...)
placebo = PlaceboTest(method='permutation', n_permutations=500).fit(data, ...)

# Subsample / specification robustness
for subsample_mask in subsamples:
    res = CS().fit(data[subsample_mask], ...)

Standard robustness battery:

• Placebo / fake timing
• Falsification on pre-reform period
• Alternative outcome variables
• Subsample splits by pre-determined characteristics
• Randomization inference

Step 8 — Heterogeneous Treatment Effects

# Triple difference for effect heterogeneity by subgroup Z
from diff_diff import DDD

ddd = DDD().fit(data, outcome='y', treatment='treated',
                time='post', third_diff='high_exposure', cluster='id')

# Interaction-based heterogeneity in TWFE
from diff_diff import TWFE
res = TWFE().fit(data, outcome='y',
                 treatment='treat_post',
                 interactions=['treat_post:firm_size'],
                 absorb=['id', 'year'], cluster='id')

Step 9 — Mechanism Analysis

1. Outcome ladder: immediate → intermediate → final outcome
2. Mediation: include proposed mechanism as control; compare ATT with/without
3. Heterogeneity as mechanism: subgroup event studies by initial conditions

Step 10 — Welfare & Policy Implications

• Aggregate ATTs to policy-level impacts
• Cost-benefit ratios
• Distributional effects (winners vs. losers)

Step 11 — Full Workflow Summary

1. Data & balance
2. Identification assumptions
3. Baseline specs M1–M6 + Oster δ*
4. Event study (TWFE + CS + SA + BJS)
5. HonestDiD sensitivity
6. Alternative explanations
7. Robustness battery
8. Heterogeneous effects (DDD / interactions)
9. Mechanisms
10. Welfare implications

---

Causal Inference: Method Selection

What is your identification strategy?
├── Policy/treatment with parallel trends → DID (see above)
├── Exogenous instrument for endogenous X → IV
├── Discontinuity in assignment rule → RD / RKD
├── Control units that can be reweighted → Synthetic Control
├── Selection on observables → Matching / IPW
└── High-dimensional / ML setting → DML / Causal Forest

---

Instrumental Variables (IV / 2SLS / GMM)

Library: linearmodels (preferred over statsmodels for panel IV)

Key assumptions: Relevance (F > 10, ideally > 104 per Lee et al. 2022), Exclusion restriction, Independence.

from linearmodels.iv import IV2SLS, IVGMM, IVLIML

# Basic 2SLS: y ~ X_exog + [X_endog ~ Z_instruments]
res = IV2SLS(dependent=y,
             exog=X_exog,        # included exogenous (+ constant)
             endog=X_endog,      # endogenous regressors
             instruments=Z).fit(cov_type='robust')

# Panel IV with fixed effects
from linearmodels import PanelOLS, BetweenOLS
from linearmodels.iv import IV2SLS
# absorb FE first (within transform), then IV on residuals
# or use linearmodels.panel with IV support

# GMM (efficient with heteroskedasticity)
res = IVGMM(y, X_exog, X_endog, Z).fit(cov_type='robust')

# LIML (less biased with weak instruments)
res = IVLIML(y, X_exog, X_endog, Z).fit(cov_type='robust')

# Key diagnostics
print(res.first_stage)          # first-stage F-statistic
print(res.wooldridge_score)     # endogeneity test (H0: OLS consistent)
print(res.sargan)               # overidentification test (J-stat, requires overid)

IV Diagnostics Checklist

Test	What it checks	Pass if
First-stage F	Instrument relevance	F > 104 (Lee et al.) or > 10 (rule of thumb)
Cragg-Donald / Kleibergen-Paap	Weak instrument (multiple endog)	> Stock-Yogo critical values
Sargan-Hansen J-test	Overidentification (exclusion)	p > 0.1 (can't reject validity)
Hausman / Wooldridge	Endogeneity of X	p < 0.05 → IV needed
Reduced form	Instrument affects outcome	Should be significant

# Anderson-Rubin confidence set (robust to weak instruments)
from linearmodels.iv import IV2SLS
res = IV2SLS(y, X_exog, X_endog, Z).fit(cov_type='robust')
print(res.anderson_rubin)  # AR test, valid even with weak instruments

# Conley spatial HAC SEs (geographic instruments)
res = IV2SLS(y, X_exog, X_endog, Z).fit(cov_type='kernel', bandwidth=5)

Bartik / Shift-Share IV

# Bartik instrument: Z_i = sum_k s_{ik} * g_k
# s_{ik}: industry share of unit i; g_k: national industry growth
import numpy as np

def bartik_instrument(shares, growth):
    """
    shares: (n_units, n_industries)
    growth: (n_industries,)
    returns: (n_units,) Bartik instrument
    """
    return shares @ growth

---

Regression Discontinuity (RD / RKD / Fuzzy RD)

Library: rdrobust (Python port of R package)

Key assumption: Units cannot precisely manipulate the running variable around the cutoff.

from rdrobust import rdrobust, rdbwselect, rdplot

# Sharp RD
res = rdrobust(y, x, c=cutoff)          # default: MSE-optimal bandwidth, local linear
res = rdrobust(y, x, c=0,
               kernel='triangular',     # triangular (default) / uniform / epanechnikov
               bwselect='mserd',        # MSE-optimal (default); 'cerrd' for coverage
               vce='hc1',              # robust SEs
               cluster=cluster_var)
print(res)

# Fuzzy RD (instrument = 1[x >= c])
res_fuzzy = rdrobust(y, x, c=0,
                     fuzzy=treatment_var)  # IV-style, estimates LATE

# Bandwidth selection
bw = rdbwselect(y, x, c=0, bwselect='mserd')
print(bw.bws)    # optimal bandwidth

# Visualization
rdplot(y, x, c=0)   # binned scatter with polynomial fit

RD Diagnostics Checklist

from rddensity import rddensity
from rdrobust import rdrobust

# 1. McCrary density test (H0: no manipulation at cutoff)
den = rddensity(x, c=cutoff)
print(den.test)   # p > 0.05: no evidence of manipulation

# 2. Covariate smoothness (placebo on pre-determined covariates)
for cov in baseline_covariates:
    res = rdrobust(cov, x, c=cutoff)
    print(f'{cov}: {res.coef[0]:.3f} (p={res.pv[2]:.3f})')  # should be insignificant

# 3. Placebo cutoffs (should find no effect at fake cutoffs)
for fake_c in [cutoff - 0.5, cutoff + 0.5]:
    res = rdrobust(y, x, c=fake_c)
    print(f'Placebo c={fake_c}: {res.coef[0]:.3f}')

# 4. Sensitivity to bandwidth
for h in [bw_opt * 0.5, bw_opt * 0.75, bw_opt, bw_opt * 1.25, bw_opt * 1.5]:
    res = rdrobust(y, x, c=cutoff, h=h)
    print(f'h={h:.2f}: {res.coef[0]:.3f}')

# 5. Donut hole (exclude units very close to cutoff)
mask = np.abs(x - cutoff) > donut_radius
res_donut = rdrobust(y[mask], x[mask], c=cutoff)

Regression Kink Design (RKD)

# RKD: identifies effect via kink (slope discontinuity) rather than level jump
res_rkd = rdrobust(y, x, c=cutoff, deriv=1)  # deriv=1 estimates slope discontinuity

---

Synthetic Control

Use when: Few treated units (often N=1), long pre-treatment panel, no obvious control group.

Libraries: pysynth, synth_control (pip), or manual implementation via scipy.optimize.

# --- Option 1: pysynth ---
from pysynth import Synth

sc = Synth()
sc.fit(
    dataprep={
        'foo_table': df,
        'predictors': ['gdp', 'trade', 'invest'],
        'predictors_op': 'mean',
        'time_predictors_prior': list(range(1980, 1990)),
        'special_predictors': [('gdp', [1985, 1988], 'mean')],
        'dependent': 'gdp',
        'unit_variable': 'country',
        'time_variable': 'year',
        'treatment_identifier': 'basque',
        'controls_identifier': control_countries,
        'time_optimize_ssr': list(range(1960, 1990)),
        'time_plot': list(range(1960, 1998)),
    }
)
sc.plot(['trends', 'weights', 'gaps'])

# --- Option 2: manual (scipy) ---
from scipy.optimize import minimize
import numpy as np

def synth_loss(w, Y_pre_control, Y_pre_treated):
    """Minimize pre-treatment fit: ||Y_treated - Y_control @ w||^2"""
    return np.sum((Y_pre_treated - Y_pre_control @ w) ** 2)

n_controls = Y_pre_control.shape[1]
w0 = np.ones(n_controls) / n_controls
constraints = [{'type': 'eq', 'fun': lambda w: w.sum() - 1}]
bounds = [(0, 1)] * n_controls

res = minimize(synth_loss, w0,
               args=(Y_pre_control, Y_pre_treated),
               method='SLSQP',
               bounds=bounds,
               constraints=constraints)
w_opt = res.x
Y_synth = Y_post_control @ w_opt
gap = Y_post_treated - Y_synth

Synthetic Control Diagnostics

# Pre-treatment fit (RMSPE)
rmspe_pre = np.sqrt(np.mean((Y_pre_treated - Y_pre_control @ w_opt)**2))

# Placebo tests: apply SC to each control unit, compute distribution of gaps
placebo_gaps = []
for ctrl in control_units:
    Y_treated_placebo = Y_pre[:, ctrl_idx]
    Y_control_placebo = np.delete(Y_pre, ctrl_idx, axis=1)
    # ... fit and store gap
    placebo_gaps.append(gap_placebo)

# Ratio: treated RMSPE_post / RMSPE_pre vs. controls (Abadie et al. 2010)
ratio_treated = rmspe_post / rmspe_pre
# Inference: fraction of placebos with ratio >= ratio_treated → p-value

# In-time placebo: apply SC using period before actual treatment as fake treatment
# In-space placebo: already done above

Synthetic DiD (SDiD)

# Combines SC weights with DiD — robust to both parallel trends violations and
# imperfect pre-treatment fit
from diff_diff import SDiD

sdid = SDiD()
res = sdid.fit(data, outcome='y', treatment='treated',
               unit='id', time='year', cluster='id')
res.print_summary()

---

Matching and Reweighting

Use when: Selection on observables; rich baseline covariate data.

Estimands: ATT (treated vs. matched controls), ATE (population average).

Propensity Score Methods

from sklearn.linear_model import LogisticRegression
from sklearn.preprocessing import StandardScaler
import numpy as np

# 1. Estimate propensity score
X_scaled = StandardScaler().fit_transform(X_covariates)
ps_model = LogisticRegression(C=1.0, max_iter=1000)
ps_model.fit(X_scaled, treatment)
p_score = ps_model.predict_proba(X_scaled)[:, 1]

# 2. Check overlap / common support
import matplotlib.pyplot as plt
plt.hist(p_score[treatment==1], alpha=0.5, label='Treated', bins=30)
plt.hist(p_score[treatment==0], alpha=0.5, label='Control', bins=30)
plt.legend(); plt.xlabel('Propensity Score')
# Trim tails: drop obs with p_score outside [0.05, 0.95]
mask = (p_score >= 0.05) & (p_score <= 0.95)

IPW / AIPW (Doubly Robust)

from econml.dr import LinearDRLearner
from sklearn.linear_model import LassoCV, LogisticRegressionCV

# Doubly robust (AIPW) — consistent if either outcome or propensity model correct
dr = LinearDRLearner(
    model_regression=LassoCV(),      # outcome model
    model_propensity=LogisticRegressionCV(),  # propensity model
    featurizer=None
)
dr.fit(Y, T, X=X_het, W=X_controls)  # X: effect modifiers, W: controls
ate = dr.ate(X_het)
print(dr.ate_interval(X_het))         # confidence interval

Nearest-Neighbor Matching

from causalml.match import NearestNeighborMatch
from causalml.propensity import ElasticNetPropensityModel

# Propensity score matching
pm = ElasticNetPropensityModel()
ps = pm.fit_predict(X_covariates, treatment)

matcher = NearestNeighborMatch(replace=False, ratio=1, random_state=42)
matched = matcher.match(data=df, treatment_col='treated', score_cols=['ps'])

# ATT on matched sample
att = matched[matched.treated==1]['y'].mean() - matched[matched.treated==0]['y'].mean()

# OR: Mahalanobis distance matching (better for low-dimensional X)
from pymatch.Matcher import Matcher
m = Matcher(test=df[df.treated==1], control=df[df.treated==0],
            yvar='y', exclude=['id'])
m.fit_scores(balance=True, nmodels=10)
m.predict_scores()
m.match(method='min', nmatches=1, threshold=0.001)
m.assess_balance(actual=True)

Covariate Balance Diagnostics

# Standardized mean differences (SMD) before/after matching
def smd(x_treat, x_control):
    return (x_treat.mean() - x_control.mean()) / np.sqrt(
        (x_treat.var() + x_control.var()) / 2
    )

for col in covariates:
    before = smd(df[df.treated==1][col], df[df.treated==0][col])
    after  = smd(matched[matched.treated==1][col], matched[matched.treated==0][col])
    print(f'{col}: SMD before={before:.3f}, after={after:.3f}')
# Target: |SMD| < 0.1 after matching

# Love plot
import matplotlib.pyplot as plt
smds_before = [...]
smds_after  = [...]
plt.scatter(smds_before, covariates, label='Before', marker='o')
plt.scatter(smds_after,  covariates, label='After',  marker='s')
plt.axvline(0, color='k', lw=0.5); plt.axvline(0.1, color='r', ls='--')
plt.legend(); plt.xlabel('Standardized Mean Difference')

Entropy Balancing

# Reweight controls to exactly match treated means (and optionally variances)
# Install: pip install ebal
from ebal import ebal

# Balances moments of X_controls exactly — no propensity model needed
weights = ebal(X_control=X[treatment==0],
               X_treated=X[treatment==1],
               moments=1)   # 1=means, 2=means+variances

# Use weights in weighted regression
import pyfixest as pf
df["w"] = np.where(treatment == 1, 1.0, weights)
res = pf.feols("y ~ treated", data=df, weights="w", vcov={"CRV1": "id"})

---

Double Machine Learning (DML) & Causal Forests

Use when: High-dimensional controls; heterogeneous treatment effects; flexible functional form.

from econml.dml import LinearDML, CausalForestDML, NonParamDML
from econml.dr import ForestDRLearner
from sklearn.ensemble import GradientBoostingRegressor, GradientBoostingClassifier
from sklearn.linear_model import LassoCV, LogisticRegressionCV

# --- Linear DML (Partially Linear Robinson model) ---
dml = LinearDML(
    model_y=LassoCV(),              # outcome residualization
    model_t=LassoCV(),              # treatment residualization
    discrete_treatment=False,
    cv=5,
)
dml.fit(Y, T, X=X_het, W=X_controls)
print(dml.ate(), dml.ate_interval())

# --- Causal Forest (nonparametric CATE) ---
cf = CausalForestDML(
    model_y=GradientBoostingRegressor(),
    model_t=GradientBoostingRegressor(),
    n_estimators=1000,
    min_samples_leaf=5,
    max_depth=5,
    discrete_treatment=False,
    cv=5,
)
cf.fit(Y, T, X=X_het, W=X_controls)

# Heterogeneous effects
tau_hat = cf.effect(X_het)            # CATE for each unit
lb, ub = cf.effect_interval(X_het)   # 95% CI

# Feature importance for heterogeneity
cf.feature_importances_  # which X drives heterogeneity

# Best linear predictor of CATE
blp = cf.ate_inference(X_het)
blp.summary_frame()

# --- IV + DML (DRIV for endogenous treatment) ---
from econml.iv.dr import LinearDRIV
driv = LinearDRIV(
    model_y_xw=LassoCV(),
    model_t_xw=LassoCV(),
    model_z=LogisticRegressionCV(),  # instrument model
    discrete_instrument=True,
)
driv.fit(Y, T, Z=Z_instrument, X=X_het, W=X_controls)

---

Causal Method Selection Guide

Setting	Method	Key Library
OLS / WLS / Poisson with FE	Linear models	pyfixest
Panel + policy shock, parallel trends	DID / TWFE / CS / SA	diff-diff + pyfixest
Staggered adoption	CS, SA, BJS	diff-diff
Exogenous instrument	2SLS / GMM / LIML	linearmodels (or pyfixest for panel IV)
Weak instrument concern	AR confidence set, LIML	linearmodels
Cutoff assignment rule	Sharp / Fuzzy RD	rdrobust
Slope discontinuity	RKD	rdrobust (deriv=1)
N=1 treated, long panel	Synthetic Control	pysynth / manual
SC + panel structure	Synthetic DiD	diff-diff (SDiD)
Selection on observables	PSM / IPW / EB	causalml, ebal
High-dim controls, binary T	AIPW / DR-Learner	econml
Heterogeneous effects	Causal Forest	econml
Endogenous T + heterogeneity	DRIV	econml

---

General Numerical Patterns

Root Finding

from scipy.optimize import brentq, root

# Scalar: prefer brentq (robust)
r_star = brentq(lambda r: asset_market_clearing(r, params), -0.05, 0.1)

# Multivariate
sol = root(equilibrium_system, x0=initial_guess, method='hybr', tol=1e-10)

Income Process Discretization

import quantecon as qe

# Tauchen: AR(1) log y' = rho log y + sigma_e * eps
mc = qe.tauchen(rho, sigma_e, n=7)
y_grid = np.exp(mc.state_values)
trans_mat = mc.P

# Rouwenhorst (better for high persistence)
mc = qe.rouwenhorst(n=7, rho=rho, sigma=sigma_e)

Performance Hierarchy

# 1. Vectorize with numpy first
# 2. Must loop → @njit
# 3. Parallelizable outer loop → @njit(parallel=True) + prange
# 4. Sparse structure → scipy.sparse

from numba import njit, prange

@njit(parallel=True)
def parallel_vfi(V, a_grid, y_grid, beta, sigma):
    n_a = len(a_grid)
    V_new = np.empty_like(V)
    for ia in prange(n_a):
        ...
    return V_new

---

Common Mistakes

Mistake	Correct Approach
TWFE with staggered treatment	Use CS / SA / BJS to avoid negative-weight bias
DID without clustered SEs	cluster='id' in diff_diff
Few clusters (<50)	inference='wild_bootstrap' in diff-diff
IV: not checking first-stage F	Always print res.first_stage; F > 104 preferred
IV: J-test p < 0.05 with overid	Instrument likely invalid; reconsider exclusion restriction
RD: single bandwidth choice	Show robustness across multiple bandwidths
RD: not testing density at cutoff	Run McCrary / rddensity test always
Matching: not checking balance	Report SMD before/after; target |SMD| < 0.1
Matching: ignoring common support	Trim p-score outside [0.05, 0.95]
SC: poor pre-treatment fit	RMSPE_pre high → SC weights unreliable; report fit explicitly
VFI inner loops without Numba	Decorate with @njit
Uniform grid for income	Tauchen / Rouwenhorst discretization
Linear asset grid	Log/exponential spacing near borrowing constraint
Not checking solver convergence	Inspect sol.success and residuals

---

Debugging Checklist

DID: Pre-period event study coefficients ≈ 0; Goodman-Bacon decomposition for TWFE weight check

IV: First-stage F > 104; reduced form significant; J-test p > 0.1 (overid); AR confidence set if weak instruments

RD: Density test p > 0.05; covariates smooth at cutoff; robust to bandwidth choice

SC: Pre-treatment RMSPE small; placebo RMSPE ratio (post/pre) for inference

Matching: |SMD| < 0.1 after matching; Love plot; common support overlap

DSGE/HANK: All market-clearing residuals < 1e-8; VFI: plot max|V_{n+1} - V_n|; Distribution: assert np.isclose(dist.sum(), 1.0); Jacobian: np.allclose(J_analytic, J_fd, rtol=1e-4)