Analyzing the Information Contained in the Skewness and Kurtosis of TEPIX Returns for Forecasting Risk: GARCH Model with Gram-Charlier Expansions for Innovations

Document Type : Original Article

Author

Assistant Professor, Nahavand Higher Education Complex, Bu-Ali Sina University, Hamedan, Iran

Abstract

Purpose: A well-known stylized fact of asset return distributions is the pattern of skewness and kurtosis. Previous studies demonstrate that financial crises and turbulence induce shocks that significantly affect return distributions, resulting in both fat tails and asymmetric tail reactions. Despite the common influence of skewness and kurtosis on tail risk, their significance for risk forecasting has been underexplored in empirical financial research. Developing models for accurate risk forecasting remains a critical focus for policymakers, economists, financial market participants, and researchers. In this study, following Jimenez et al. (2022), a semi-nonparametric approach is adopted to estimate return densities based on the asymptotic properties of Gram-Charlier expansions. This approach allows for evaluating the significance of incorporating Hermite polynomials and their cross-products into Gram-Charlier densities for risk forecasting. Using this semi-nonparametric framework, risk measures can capture all stylized facts of the return series by incorporating new parameters, enabling a comprehensive assessment of skewness, kurtosis, and their interactions as valuable sources of information.
Method: For the first time, this study employs the modified Gram-Charlier (mGC) density function, which includes second and third moments (skewness and kurtosis) and their interactions, to model the risk distribution of daily losses in the TEPIX index. The performance of alternative models based on different Gram-Charlier specifications is evaluated in terms of risk forecasting accuracy using advanced backtesting tests. Specifically, the Value-at-Risk (VaR) criteria and the Median Shortfall (MS) measure—introduced for the first time in this research—are utilized. The sample comprises daily TEPIX index series from May 20, 2008, to August 22, 2023. The loss series is computed as the negative logarithmic differences of prices, focusing on the right tail of the TEPIX distribution. Estimations are conducted using R and MATLAB software through a two-step process:
-The ARMA(1,1)-GARCH(1,1) model is estimated using the quasi-maximum likelihood (QML) approach, assuming Gaussian distribution for the error terms.
-The modified Gram-Charlier expansion and its alternative specifications are estimated using standardized residuals extracted from the first step, with maximum likelihood estimation applied for density fitting. The in-sample estimation window consists of 2,656 observations, and forecasts are updated with one new observation. The remaining 1,000 observations are used for out-of-sample forecasting.
Findings: The in-sample fitting of the ARMA(1,1)-GARCH(1,1) model, under the Gram-Charlier densities for innovations, indicates that skewness, kurtosis, and their interactions are statistically and economically significant. Backtesting results for 99%-VaR and 99%-MS demonstrate that the Gram-Charlier density specification incorporating the skewness parameter significantly improves out-of-sample risk forecasting, particularly for the tails, compared to alternative specifications.
Conclusion: Overall, the findings reveal that incorporating the asymmetry parameter of the return density as a stand-alone feature provides a valuable source of information, enabling accurate risk measures for market participants. These results have practical implications for designing risk management strategies and decision-making under market instability. The novelty of this study lies in applying a semi-nonparametric approach to evaluate risk forecasting for the TEPIX index. Unlike previous studies that relied primarily on parametric and non-parametric distributions, this research offers an innovative application for risk management in the Tehran Stock Exchange. The empirical findings also have potential implications for stabilizing financial markets.

Keywords


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