Left panel: X-ray flux distribution. From top to bottom, black line: all X-ray sources found in 2XMMi fields overlapping SDSS DR7, red line: all X-ray sources matching a SDSS entry, red dashed line: 2XMMi/SDSS R7 correlations with individual identification probabilities  >90%, blue line: “final” identified sample. Centre panel: ratio of the SDSS identified to total number of 2XMMi sources. Upper curve, filled squares; all matches, lower curve, empty squares, matches with identification probabilities  ≥90%. Right panel: variation of log(f_x/f_r) with X-ray flux for 2XMMi/SDSS DR7 sources with identification probabilities  ≥ 90% and r magnitude brighter than 22.2. In all cases, the combined unique EPIC source detection likelihood is  ≥ 6.