Credible contours for the logarithmic X-ray/radio ratio q. The Bayes formula was used to derive the posterior probability for q, shown as contours (levels relative to 50%, 68.3% and 90%). The blue continuous and the green long-dashed contours refer to SF objects with z ≤ 0.2 and z > 0.2, respectively. The red short-dashed contours refer to SF objects at any redshift (i.e., the sum of the two bins). The green long-dashed arrow shows the correction that should be made to q for the z > 0.2 and any-z bins to account for the bias due to radio selection. The grey-scale background shows the prior (a darker grey means a higher probability density). The yellow asterisk shows the q₀ value from RCS03.