Monochromatic 6 μm luminosities (L_6 μm) of red QSOs (red filled circles), control QSOs (sky-blue filled circles) and Lz-matched control QSOs (dark-blue circles) as a function of redshift are plotted in the central panel. Side panels show normalised histograms of the L_6 μm and redshift distributions for each population, shown as solid lines (red QSO), dashed lines (L − z matched samples) and shaded areas (entire control sample) of respective colours. By comparing the distributions of the red QSOs and the Lz-matched control QSO sample, it can be clearly seen that equivalent QSO properties are being sampled.