Normalised distributions of radio-loudness parameter ℛ, derived as described in Sect. 3; the error bars reported represent the mean uncertainties obtained as detailed in Sect. 3.4. Left panel: radio-detected (green shaded histogram) and radio-undetected (red dotted line) sources in the “main sample”. Central panel: ROSAT-detected (brown shaded histogram) and ROSAT-undetected (black continuous line) sources in the “control sample”. Right panel: radio-detected sources in the “main sample”, divided between ROSAT-detected (brown shaded histogram) and ROSAT-undetected (black continuous line).