Use this Skill for multivariate time series: VAR, Granger causality, Johansen cointegration, VECM, impulse response functions, and forecast error variance decomposition.
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Use this Skill for multivariate time series: VAR, Granger causality, Johansen cointegration, VECM, impulse response functions, and forecast error variance decomposition.
TL;DR — Fit VAR models, test Granger causality, test for cointegration with
Johansen, estimate VECMs, plot impulse response functions with bootstrap confidence
intervals, and decompose forecast error variance.
Non-stationary series that may share long-run trends
Johansen cointegration test
Cointegrated system with error correction
VECM
Dynamic response of one variable to a shock in another
Impulse Response Functions (IRF)
How much of Y's forecast variance is explained by X?
Forecast Error Variance Decomposition (FEVD)
This Skill is the starting point for any empirical work involving multiple time series
in economics and finance — GDP, inflation, interest rates, exchange rates, commodity
prices, and asset prices.
where u_t ~ N(0, Σ) is the innovation vector. Lag p is selected by information
criteria: AIC, BIC (Schwarz), or HQIC.
VAR stability condition: All eigenvalues of the companion matrix must lie strictly
inside the unit circle. If any eigenvalue >= 1, the system is non-stationary and
you should consider VECM.
Granger Causality
Variable X Granger-causes Y if knowing past X improves forecasts of Y beyond past Y
alone. Tested via an F-test on the restriction that all lagged X coefficients in the
Y equation are jointly zero.
This is a predictive concept, not a structural causal one. Granger causality ≠
structural causality.
Johansen Cointegration
Two or more I(1) series are cointegrated if a linear combination is I(0). Johansen
(1988) provides two likelihood-ratio tests:
Trace test: H0 = at most r cointegrating vectors
Maximum eigenvalue test: H0 = r vs H1 = r+1 cointegrating vectors
Critical values differ by deterministic specification (no constant, constant in CE,
trend in CE). Use statsmodels.tsa.vector_ar.vecm.coint_johansen.
VECM
A VECM re-parameterizes the VAR for I(1) cointegrated variables:
where Π = αβ' is the error correction matrix, β is the cointegrating vector,
and α is the speed-of-adjustment vector.
Impulse Response Functions
The IRF measures the dynamic response of Y_{t+h} to a one-unit shock in u_{j,t}.
Cholesky decomposition of Σ identifies structural shocks via a lower-triangular
ordering (Cholesky ordering matters for interpretation).
FEVD
FEVD at horizon h decomposes the forecast variance of variable i into contributions
from each structural shock j. FEVD_{ij}(h) ∈ [0, 1] and Σ_j FEVD_{ij}(h) = 1.
import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
from statsmodels.tsa.stattools import adfuller, kpss
from statsmodels.tsa.vector_ar.var_model import VAR
from statsmodels.tsa.vector_ar.vecm import coint_johansen, VECM
import warnings
np.random.seed(42)
defadf_summary(series: pd.Series, name: str = "", max_diff: int = 2) -> pd.DataFrame:
"""
Run ADF test on a series and its differences until stationarity is achieved.
Args:
series: Time series to test.
name: Label for display.
max_diff: Maximum order of differencing to attempt.
Returns:
DataFrame with columns: variable, adf_stat, p_value, lags, conclusion.
"""
records = []
for d inrange(max_diff + 1):
s = series.diff(d).dropna() if d > 0else series.dropna()
label = f"Δ^{d}{name}"if d > 0else name
adf_result = adfuller(s, autolag="AIC")
stat, pval, lags_used = adf_result[0], adf_result[1], adf_result[2]
conclusion = "stationary (I(0))"if pval < 0.05else"unit root (non-stationary)"
records.append({
"variable": label,
"adf_stat": round(stat, 4),
"p_value": round(pval, 4),
"lags": lags_used,
"conclusion": conclusion,
})
if pval < 0.05:
breakreturn pd.DataFrame(records)
defgenerate_var_data(
n: int = 300,
k: int = 3,
p: int = 2,
cointegrated: bool = False,
) -> pd.DataFrame:
"""
Simulate a stationary VAR(p) or cointegrated I(1) system.
Args:
n: Number of observations.
k: Number of variables.
p: Lag order of the true VAR.
cointegrated: If True, generate cointegrated I(1) system.
Returns:
DataFrame with k columns and n rows.
"""ifnot cointegrated:
# Stationary VAR: companion matrix has spectral radius < 1
A1 = np.array([[0.5, 0.1, 0.0],
[0.0, 0.4, 0.2],
[0.1, 0.0, 0.3]])
A2 = np.array([[0.1, 0.0, 0.0],
[0.0, 0.1, 0.0],
[0.0, 0.0, 0.1]])
Sigma = np.array([[1.0, 0.3, 0.1],
[0.3, 1.0, 0.2],
[0.1, 0.2, 1.0]])
L = np.linalg.cholesky(Sigma)
Y = np.zeros((n + 50, k))
for t inrange(2, n + 50):
eps = L @ np.random.randn(k)
Y[t] = A1 @ Y[t - 1] + A2 @ Y[t - 2] + eps
Y = Y[50:]
cols = [f"y{i+1}"for i inrange(k)]
return pd.DataFrame(Y, columns=cols)
else:
# I(1) cointegrated: three series share one common stochastic trend
common_trend = np.cumsum(np.random.randn(n))
Y = np.column_stack([
common_trend + 0.5 * np.random.randn(n),
2 * common_trend + np.random.randn(n),
-0.5 * common_trend + 1.5 * np.random.randn(n),
])
cols = ["x1", "x2", "x3"]
return pd.DataFrame(Y, columns=cols)
Step 2 — VAR Estimation and Granger Causality
deffit_var_and_granger(
df: pd.DataFrame,
maxlags: int = 8,
ic: str = "aic",
verbose: bool = True,
) -> dict:
"""
Fit a VAR(p) model, run stability check, and test pairwise Granger causality.
Args:
df: DataFrame of stationary time series (k columns).
maxlags: Maximum lag order to consider.
ic: Information criterion for lag selection: 'aic', 'bic', or 'hqic'.
verbose: Print results to console.
Returns:
Dictionary with keys: model, results, lag_order, granger_table, stable.
"""
model = VAR(df)
# Lag selection
lag_selection = model.select_order(maxlags=maxlags)
if verbose:
print(lag_selection.summary())
# Fit at IC-optimal lag
p_opt = getattr(lag_selection, ic)
p_opt = max(1, p_opt) # ensure at least 1 lag
results = model.fit(p_opt)
if verbose:
print(results.summary())
# Stability check
eigenvalues = np.abs(results.roots)
stable = bool(np.all(eigenvalues > 1)) # roots of char polynomial outside unit circleif verbose:
print(f"\nVAR stability: {'STABLE'if stable else'UNSTABLE'}")
print(f" Min |root| = {eigenvalues.min():.4f} (must be > 1)")
# Granger causality: all pairwise tests
variables = df.columns.tolist()
granger_rows = []
for caused in variables:
for causing in variables:
if caused == causing:
continuewith warnings.catch_warnings():
warnings.simplefilter("ignore")
gc_test = results.test_causality(caused, causing, kind="f")
granger_rows.append({
"H0": f"{causing} does NOT Granger-cause {caused}",
"F_stat": round(gc_test.test_statistic, 4),
"p_value": round(gc_test.pvalue, 4),
"reject_H0": gc_test.pvalue < 0.05,
})
granger_table = pd.DataFrame(granger_rows)
if verbose:
print("\nGranger Causality Tests:")
print(granger_table.to_string(index=False))
return {
"model": model,
"results": results,
"lag_order": p_opt,
"granger_table": granger_table,
"stable": stable,
}
defplot_irf_fevd(
var_results,
periods: int = 12,
signif: float = 0.05,
output_prefix: str = "var",
) -> None:
"""
Plot impulse response functions and FEVD for a fitted VAR.
Args:
var_results: Fitted VAR results object (statsmodels VARResults).
periods: IRF horizon in periods.
signif: Significance level for bootstrap CI (default 0.05 = 95%).
output_prefix: Prefix for saved figure filenames.
"""
irf = var_results.irf(periods)
k = var_results.neqs
variables = var_results.names
# IRF plot
fig_irf = irf.plot(
orth=True, # Cholesky orthogonalization
signif=signif,
figsize=(12, 9),
)
fig_irf.suptitle(f"Orthogonalized IRFs (Cholesky ordering: {', '.join(variables)})")
fig_irf.tight_layout()
fig_irf.savefig(f"{output_prefix}_irf.png", dpi=150)
print(f"Saved IRF plot to {output_prefix}_irf.png")
# FEVD at horizons 1, 4, 8, 12
fevd = var_results.fevd(periods)
fig_fevd = fevd.plot(figsize=(12, 6))
fig_fevd.suptitle("Forecast Error Variance Decomposition")
fig_fevd.tight_layout()
fig_fevd.savefig(f"{output_prefix}_fevd.png", dpi=150)
print(f"Saved FEVD plot to {output_prefix}_fevd.png")
# Print FEVD table at selected horizonsprint("\nFEVD at horizons 1 / 4 / 8 / 12:")
for h in [1, 4, 8, 12]:
if h <= periods:
print(f"\n Horizon {h}:")
fevd_df = pd.DataFrame(fevd.decomp[h - 1],
index=variables, columns=variables)
print(fevd_df.round(3).to_string())
Step 3 — Johansen Test and VECM Estimation
defjohansen_vecm_workflow(
df_levels: pd.DataFrame,
det_order: int = 0,
k_ar_diff: int = 1,
verbose: bool = True,
) -> dict:
"""
Run Johansen cointegration test and estimate VECM.
Args:
df_levels: DataFrame of I(1) series in levels (not differenced).
det_order: Deterministic terms: -1 (none), 0 (constant), 1 (linear trend).
k_ar_diff: Lag order in the VECM (lags of differences).
verbose: Print results.
Returns:
Dictionary with keys: johansen_result, r_selected, vecm_results, beta, alpha.
"""# Step 1: Johansen test
johansen = coint_johansen(df_levels.values, det_order, k_ar_diff)
variables = df_levels.columns.tolist()
k = len(variables)
if verbose:
print("=" * 60)
print("Johansen Cointegration Test")
print("=" * 60)
trace_cv = johansen.cvt # (k, 3): 90%, 95%, 99% critical values
maxeig_cv = johansen.cvm
print("\nTrace Test:")
print(f"{'H0: r<=':>12}{'Trace Stat':>12}{'CV 5%':>10}{'Reject?':>8}")
for r inrange(k):
stat = johansen.lr1[r]
cv5 = trace_cv[r, 1]
reject = stat > cv5
print(f" r <= {r:>4}{stat:>12.4f}{cv5:>10.4f}{'YES'if reject else'NO':>8}")
print("\nMax-Eigenvalue Test:")
print(f"{'H0: r=':>12}{'Max-Eig Stat':>12}{'CV 5%':>10}{'Reject?':>8}")
for r inrange(k):
stat = johansen.lr2[r]
cv5 = maxeig_cv[r, 1]
reject = stat > cv5
print(f" r = {r:>5}{stat:>12.4f}{cv5:>10.4f}{'YES'if reject else'NO':>8}")
# Select cointegrating rank r
r_selected = 0for r inrange(k):
if johansen.lr1[r] > johansen.cvt[r, 1]:
r_selected = r + 1if verbose:
print(f"\nSelected cointegrating rank: r = {r_selected}")
if r_selected == 0:
print("No cointegration found — estimate VAR in differences.")
return {"johansen_result": johansen, "r_selected": 0,
"vecm_results": None, "beta": None, "alpha": None}
# Step 2: VECM estimation
vecm_model = VECM(df_levels, k_ar_diff=k_ar_diff, coint_rank=r_selected,
deterministic="co") # constant in cointegrating equation
vecm_result = vecm_model.fit()
if verbose:
print("\nVECM Results:")
print(vecm_result.summary())
beta = vecm_result.beta # (k, r) cointegrating vectors
alpha = vecm_result.alpha # (k, r) speed of adjustmentif verbose:
print("\nCointegrating Vector(s) [normalized]:")
beta_df = pd.DataFrame(beta, index=variables,
columns=[f"CE{i+1}"for i inrange(r_selected)])
print(beta_df.round(4))
print("\nSpeed of Adjustment (α):")
alpha_df = pd.DataFrame(alpha, index=variables,
columns=[f"CE{i+1}"for i inrange(r_selected)])
print(alpha_df.round(4))
return {
"johansen_result": johansen,
"r_selected": r_selected,
"vecm_results": vecm_result,
"beta": beta,
"alpha": alpha,
}
Advanced Usage
Bootstrap Confidence Intervals for IRF
defbootstrap_irf(
var_results,
periods: int = 12,
n_boot: int = 500,
signif: float = 0.05,
seed: int = 0,
) -> dict:
"""
Bootstrap percentile confidence intervals for Cholesky IRF.
Uses residual bootstrap: resample VAR residuals with replacement,
simulate new data, refit VAR, compute IRF.
Args:
var_results: Fitted VARResults object.
periods: IRF horizon.
n_boot: Number of bootstrap replications.
signif: Two-sided CI level (0.05 => 95% CI).
seed: Random seed for reproducibility.
Returns:
Dictionary with keys: irf_point, irf_lower, irf_upper.
Each has shape (periods+1, k, k).
"""
rng = np.random.default_rng(seed)
k = var_results.neqs
p = var_results.k_ar
residuals = var_results.resid # (T - p, k)
T_eff = residuals.shape[0]
# Centered residuals
resid_centered = residuals - residuals.mean(axis=0)
Y_orig = var_results.endog # (T, k)
coefs = var_results.coefs # (p, k, k)
intercept = var_results.intercept # (k,)
irf_boots = np.zeros((n_boot, periods + 1, k, k))
for b inrange(n_boot):
# Resample residuals
idx = rng.integers(0, T_eff, size=T_eff)
boot_resid = resid_centered[idx]
# Simulate new series
Y_boot = np.zeros((p + T_eff, k))
Y_boot[:p] = Y_orig[:p]
for t inrange(p, p + T_eff):
Y_boot[t] = intercept.copy()
for lag inrange(p):
Y_boot[t] += coefs[lag] @ Y_boot[t - lag - 1]
Y_boot[t] += boot_resid[t - p]
df_boot = pd.DataFrame(Y_boot[p:], columns=var_results.names)
with warnings.catch_warnings():
warnings.simplefilter("ignore")
try:
var_boot = VAR(df_boot).fit(p)
irf_boot = var_boot.irf(periods)
irf_boots[b] = irf_boot.orth_irfs
except Exception:
irf_boots[b] = np.nan
alpha_lo = signif / 2
alpha_hi = 1 - signif / 2
irf_lower = np.nanquantile(irf_boots, alpha_lo, axis=0)
irf_upper = np.nanquantile(irf_boots, alpha_hi, axis=0)
irf_point = var_results.irf(periods).orth_irfs
return {"irf_point": irf_point, "irf_lower": irf_lower, "irf_upper": irf_upper}
defplot_bootstrap_irf(boot_dict: dict, variable_names: list, output_path: str = None) -> None:
"""
Plot point-estimate IRF with bootstrap CI bands.
Args:
boot_dict: Output of bootstrap_irf().
variable_names: List of variable names.
output_path: If provided, save figure here.
"""
k = len(variable_names)
irf_point = boot_dict["irf_point"]
irf_lower = boot_dict["irf_lower"]
irf_upper = boot_dict["irf_upper"]
H = irf_point.shape[0]
horizons = np.arange(H)
fig, axes = plt.subplots(k, k, figsize=(4 * k, 3 * k), sharex=True)
for i inrange(k):
for j inrange(k):
ax = axes[i, j]
ax.plot(horizons, irf_point[:, i, j], color="#2980B9", linewidth=2)
ax.fill_between(horizons, irf_lower[:, i, j], irf_upper[:, i, j],
alpha=0.2, color="#2980B9")
ax.axhline(0, color="black", linewidth=0.8, linestyle="--")
if i == 0:
ax.set_title(f"Shock: {variable_names[j]}", fontsize=9)
if j == 0:
ax.set_ylabel(f"Response: {variable_names[i]}", fontsize=9)
fig.suptitle("Bootstrap IRF (95% CI)")
fig.tight_layout()
if output_path:
fig.savefig(output_path, dpi=150)
print(f"Saved bootstrap IRF to {output_path}")
plt.show()
Troubleshooting
Error / Issue
Cause
Resolution
MissingDataError
NaN values in time series
Forward-fill or drop NaN rows before fitting
VAR eigenvalue >= 1
Non-stationary system
First-difference data or use VECM
Johansen trace test selects r=k
Spurious cointegration
Check for structural breaks; verify I(1) order
VECM alpha near zero
No error correction
Re-examine cointegrating rank selection
Granger test F-stat = 0
Lag order = 1 but variable not in equation
Increase maxlags; verify variable is in system
IRF does not decay to zero
VAR is near-non-stationary
Re-test lag selection; check for unit roots
LinAlgError in Cholesky
Σ not positive definite due to collinearity
Drop redundant variables or add jitter
Very wide bootstrap CI bands
Short sample or many variables
Reduce variables; increase sample size
External Resources
Lütkepohl, H. (2005). New Introduction to Multiple Time Series Analysis. Springer.
Sims, C.A. (1980). "Macroeconomics and Reality." Econometrica, 48(1), 1–48.
Johansen, S. (1988). "Statistical Analysis of Cointegration Vectors." JEDS, 12(2–3), 231–254.
# Generate stationary data
df_stat = generate_var_data(n=400, k=3, cointegrated=False)
# Check stationarityfor col in df_stat.columns:
print(adf_summary(df_stat[col], name=col).to_string(index=False))
# Fit VAR and test Granger causality
var_out = fit_var_and_granger(df_stat, maxlags=8, ic="aic")
# IRF and FEVD plots
plot_irf_fevd(var_out["results"], periods=12, output_prefix="var_example")
Example 2 — Cointegrated System: Johansen + VECM
# Generate cointegrated I(1) data
df_coint = generate_var_data(n=400, k=3, cointegrated=True)
# Confirm unit roots in levels, stationarity in differencesfor col in df_coint.columns:
print(adf_summary(df_coint[col], name=col).to_string(index=False))
# Johansen test and VECM
vecm_out = johansen_vecm_workflow(df_coint, det_order=0, k_ar_diff=2)
# Confirm error-correction spreads are stationaryif vecm_out["beta"] isnotNone:
beta = vecm_out["beta"]
spread = df_coint.values @ beta[:, 0]
adf_spread = adfuller(spread, autolag="AIC")
print(f"\nSpread ADF p-value: {adf_spread[1]:.4f} (should be < 0.05)")
Changelog
Version
Date
Change
1.0.0
2026-03-18
Initial release — VAR, Granger, Johansen, VECM, IRF, FEVD, bootstrap CI
