The velocity shift as a function of L_Bol (a) and α_OX (b), for the WISSH sample (blue and red stars) compared to radio quiet type-1 QSOs from Wu et al. (2009; black circles) and WLQs from Wu et al. (2011) and Luo et al. (2015; orange circles). c) Residuals plot of –L_Bol and α_OX –L_Bol relations. d) Residuals plot of –α_OX and L_Bol –α_OX relations. The black lines represent the best linear fits to the data and the purple lines correspond to the 68% confidence interval.