tsai-scoretest

Score Test for First-Order Autoregressive Model with Heteroscedasticity

A Python implementation of the score test proposed by Tsai (1986) for simultaneous testing of independence and homoscedasticity in the first-order autoregressive model with nonconstant variance.

Reference

Tsai, C.-L. (1986). Score test for the first-order autoregressive model with heteroscedasticity. Biometrika, 73(2), 455-460. DOI: 10.1093/biomet/73.2.455

Installation

From PyPI (recommended)

pip install tsai-scoretest

From source

git clone https://github.com/merwanroudane/scoretest.git
cd scoretest
pip install -e .

Mathematical Framework

Model Specification

The basic model (Equation 2.1):

$$y_t = x_t'\beta + u_t \quad (t = 1, \ldots, T)$$

where $y_t$ is the observable response, $x_t$ is a known nonstochastic vector (dimension $1 \times p$), $\beta$ is a $p \times 1$ vector of unknown parameters, and $u_t$ is the unobservable random error.

AR(1) Error Process

The error follows a first-order autoregressive process (Equation 2.2):

$$u_t = \rho u_{t-1} + e_t \quad (t = 2, \ldots, T)$$

where $u_1 = e_1$, $\rho$ is the autocorrelation coefficient, and the $e_t$'s are normally and independently distributed with mean 0.

Heteroscedasticity Structure

The variance of innovations (Equation 2.3):

$$\text{var}(e_t) = w_t \sigma^2 = w(z_t, \lambda)\sigma^2$$

where $\lambda = (\lambda_1, \ldots, \lambda_q)'$, $z_t = (z_{t1}, \ldots, z_{tq})'$ is a known vector, and there exists a unique $\lambda^$ such that $w(z_t, \lambda^) = 1$ for all $t$.

Hypothesis

Null hypothesis: $H_0: \rho = 0$ and $\lambda = \lambda^*$ (no autocorrelation and homoscedasticity)

Alternative: $H_1: \rho \neq 0$ or $\lambda \neq \lambda^*$

Score Test Statistic

The joint score test statistic (Equation 2.4):

$$S = S_1 + S_2$$

where:

• $S_1 = \frac{(T\hat{\rho})^2}{T-1}$ tests $\rho = 0$
• $S_2 = \frac{1}{2}V'\bar{D}(\bar{D}'\bar{D})^{-1}\bar{D}'V$ tests $\lambda = \lambda^*$

The estimated autocorrelation (Equation 2.5):

$$\hat{\rho} = \frac{\sum_{t=2}^{T} \hat{e}t \hat{e}{t-1}}{\sum_{t=1}^{T} \hat{e}_t^2}$$

Asymptotic Distributions

Under $H_0$:

• $S_1 \sim \chi^2(1)$
• $S_2 \sim \chi^2(q)$
• $S \sim \chi^2(q+1)$

Quick Start

Basic Usage

import numpy as np
from scoretest import score_test_joint, TsaiScoreTest

# Generate sample data
np.random.seed(42)
T = 100
X = np.column_stack([np.ones(T), np.random.randn(T)])
beta = np.array([1.0, 0.5])
y = X @ beta + np.random.randn(T)

# Perform the joint score test
result = score_test_joint(y, X)
print(result)

Output:

======================================================================
Score Test for AR(1) Model with Heteroscedasticity
Tsai (1986) - Biometrika, 73(2), 455-460
======================================================================

Sample Size (T): 100
Heteroscedasticity Parameters (q): 1

----------------------------------------------------------------------
Estimated Parameters Under H₀
----------------------------------------------------------------------
  Autocorrelation (ρ̂):          0.012345
  Error Variance (σ̂²):          0.987654

----------------------------------------------------------------------
Score Test Statistics
----------------------------------------------------------------------

  Test Component         Statistic    df      P-value    Decision
  --------------------------------------------------------------
  S₁ (Autocorrelation)       0.1234     1      0.7254    
  S₂ (Heteroscedast.)        0.5678     1      0.4512    
  --------------------------------------------------------------
  S (Joint Test)             0.6912     2      0.7079    

  Significance codes: *** p<0.01, ** p<0.05, * p<0.10

----------------------------------------------------------------------
Hypothesis Testing
----------------------------------------------------------------------
  H₀: ρ = 0 and λ = λ* (No autocorrelation and homoscedasticity)
  H₁: ρ ≠ 0 or λ ≠ λ* (Presence of autocorrelation or heteroscedasticity)

  Decision at α = 0.05: Fail to Reject H₀

======================================================================
Reference: Tsai, C.-L. (1986). Biometrika, 73(2), 455-460.
======================================================================

Class-Based Interface

from scoretest import TsaiScoreTest

# Create test instance with heteroscedasticity variables
Z = np.arange(1, T + 1).reshape(-1, 1)  # Time trend
test = TsaiScoreTest(y, X, Z=Z)

# Fit and get results
result = test.fit()

# Access individual components
print(f"S1 (autocorrelation): {result.S1:.4f}")
print(f"S2 (heteroscedasticity): {result.S2:.4f}")
print(f"S (joint): {result.S:.4f}")
print(f"p-value: {result.p_value:.4f}")

Individual Tests

from scoretest import score_test_autocorrelation, score_test_heteroscedasticity

# Test for autocorrelation only
S1, p_value_S1 = score_test_autocorrelation(y, X)
print(f"Autocorrelation test: S1 = {S1:.4f}, p = {p_value_S1:.4f}")

# Test for heteroscedasticity only
Z = np.arange(1, T + 1).reshape(-1, 1)
S2, p_value_S2, df = score_test_heteroscedasticity(y, X, Z)
print(f"Heteroscedasticity test: S2 = {S2:.4f}, p = {p_value_S2:.4f}, df = {df}")

Advanced Features

Custom Weight Functions

from scoretest import TsaiScoreTest
from scoretest.weight_functions import exponential_weight, linear_weight

# Use exponential weight function (default)
test = TsaiScoreTest(y, X, Z=Z)

# Custom weight function
def custom_weight(z, lam):
    return np.exp(z @ lam)

def custom_weight_deriv(z, lam):
    return z * np.exp(z @ lam).reshape(-1, 1)

test_custom = TsaiScoreTest(
    y, X, Z=Z,
    weight_func=custom_weight,
    weight_deriv=custom_weight_deriv
)

Simulation for Critical Values

from scoretest.simulation import simulate_critical_values

# Monte Carlo simulation for finite-sample critical values
cv_results = simulate_critical_values(
    T=50, p=2, q=1,
    n_simulations=10000,
    alpha_levels=[0.01, 0.05, 0.10]
)

print(cv_results.to_latex_table())

Diagnostic Tools (Section 3 of the paper)

from scoretest.diagnostics import normal_curvature, parameter_sensitivity

# Compute normal curvature (Equation 3.2)
curvature = normal_curvature(y, X, Z)
print(f"Maximum curvature: {curvature.C_max:.4f}")

# Compute parameter sensitivity (Equation 3.4)
sensitivity = parameter_sensitivity(y, X, Z)
print(f"Sensitivity matrix:\n{sensitivity.sensitivity_matrix}")

API Reference

Main Functions

Diagnostic Functions

Simulation Functions

Robustness Properties

From Tsai (1986):

1. S₁ is robust to heteroscedasticity when $\hat{\rho}(\lambda - \lambda^*) \approx 0$ and $\lambda$ is the true parameter in the weight function.

2. S₂ is robust to autocorrelation if $\rho/\sigma^2 \approx 0$ where $\rho$ is the true autocorrelation.

Dependencies

• numpy >= 1.20.0
• scipy >= 1.7.0

Testing

# Run tests
pytest tests/

# Run with coverage
pytest tests/ --cov=scoretest --cov-report=html

Citation

If you use this package in your research, please cite both the original paper and this implementation:

@article{tsai1986score,
  title={Score test for the first-order autoregressive model with heteroscedasticity},
  author={Tsai, Chih-Ling},
  journal={Biometrika},
  volume={73},
  number={2},
  pages={455--460},
  year={1986},
  publisher={Oxford University Press}
}

@software{roudane2024tsaiscoretest,
  title={tsai-scoretest: Python implementation of Tsai (1986) score test},
  author={Roudane, Merwan},
  year={2024},
  url={https://github.com/merwanroudane/scoretest}
}

Related Tests

• Durbin-Watson Test: Tests for first-order autocorrelation but with distribution depending on design matrix
• Cook-Weisberg Test: Tests for heteroscedasticity only (special case of S₂)
• Breusch-Pagan Test: Alternative test for heteroscedasticity
• Breusch-Godfrey Test: LM test for autocorrelation of higher order

License

MIT License - see LICENSE for details.

Author

Dr Merwan Roudane

• Email: merwanroudane920@gmail.com
• GitHub: https://github.com/merwanroudane/scoretest

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

Changelog

Version 1.0.0 (2024)

• Initial release
• Complete implementation of Tsai (1986) score test
• Joint test for autocorrelation and heteroscedasticity
• Individual test components (S₁ and S₂)
• Diagnostic tools (normal curvature, parameter sensitivity)
• Monte Carlo simulation for critical values
• Multiple weight function specifications
• Comprehensive test suite
• Publication-ready output formatting