Wang, Jying-Nan, et al. “How Useful Are the Various Volatility Estimators for Improving GARCH-Based Volatility Forecasts? Evidence from the Nasdaq-100 Stock Index”. International Journal of Economics and Financial Issues, vol. 4, no. 3, July 2014, pp. 651-6, https://econjournals.com/index.php/ijefi/article/view/863.