We propose a hybrid methodology that integrates forest-based machine learning with long-memory models of stock return volatility. Our approach exploits cross-sectional information in a panel of stocks and allows time-varying parameters that are modeled as nonparametric functions of both firm-specific information and changing market conditions. Empirical results on a panel of 1,131 stocks reveal that covariates such as the VIX, idiosyncratic volatility, and momentum have heterogeneous and economically significant effects on the time-varying parameters. In terms of forecasting accuracy, our hybrid approach outperforms a broad range of time-series models and other machine-learning methods over multiple horizons and volatility regimes. Economically, our enhanced risk forecasts yield higher utility for volatility-managed stock investments and deliver minimum-variance portfolios with significantly lower return volatility and higher Sharpe ratios.
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