“…• nine variants of three parsimonious autoregressive model structures with exogenous variables (ARX): one originally proposed by Misiorek et al [19] and later used in a number of EPF studies [13,18,[20][21][22][23][24][25][26][27], one which evolved from it during the successful participation of TEAM POLAND in the Global Energy Forecasting Competition 2014 (GEFCom2014; see [28][29][30]) and an extension of the former, which creates a stronger link with yesterday's prices and additionally considers a second exogenous variable (zonal load or wind power), • three two-year long, hourly resolution test periods from three distinct power markets (GEFCom2014, Nord Pool and the U.K.), • nine variants of five classes of selection and shrinkage procedures: single-step elimination of insignificant predictors (without or with constraints), stepwise regression (with forward selection or backward elimination), ridge regression, lasso and three elastic nets (with α = 0.25, 0.5 or 0.75), • model validation in terms of the robust weekly-weighted mean absolute error (WMAE; see [1]) and the Diebold-Mariano (DM; see [31]) test and draw statistically-significant conclusions of high practical value. The remainder of the paper is structured as follows.…”