Ratio of L_[CII]/L_FIR as a function of L_FIR for J1015+0020, compared to high-z QSOs from literature (see Sect. 4 for details) with relative errors. For sources whose uncertainty on L_FIR was not available, we assume the average value of the sample. J1015+0020 is indicated by the magenta star, while blue(green) symbols refer to z ≳ 6 (z ∼ 4.5) QSOs. The best fit relation obtained from orthogonal linear regression and its 1σ error are shown by the dashed line and shaded area.