Radio loudness and equivalent width of Mg ii (left) and C iv (right). Top: radio-loudness parameter R (filled red squares: rWLQ sample, magenta framed filled squares: rWLQ-EWS sample, large open red circle with error bars: mean value and 1σ error of rWLQs, blue plus signs: comparison sample, contours: all quasars from the Shen catalogue with bal_flag = 0). Middle: proportion f_r of FIRST-detected quasars in EW bins (filled red squares: rWLQ sample, magenta framed filled square: mean value from the rWLQ-EWS sample, open blue squares: comparison sample, black lozenge: WLQ Lit (z< 3) sample, black asterisks: all quasars from the Shen catalogue with bal_flag = 0, horizontal bars: binning intervals, vertical bars: uncertainties from Poisson statistics and error propagation). Bottom: histogram distribution of the EW (solid red: rWLQ sample, dashed blue: comparison sample).