Hasantash, Gholam Hossein, et al. “Modelling the Errors of EIA’s Oil Prices and Production Forecasts by the Grey Markov Model”. International Journal of Economics and Financial Issues, vol. 2, no. 3, June 2012, pp. 312-9, https://mail.econjournals.com/index.php/ijefi/article/view/212.