Areas covered by the XMM-LSS (black solid curve), XMM-CDFS (red long-dashed curve), and XMM-COSMOS (green short-dashed curve) surveys, vs. 2–10 keV flux. Although the nominal depth of LSS is 10 ks, it also includes the Subaru Deep Field (Ueda et al. 2008) whose exposure is 100 ks. The blue dotted curve shows the LSS coverage after including selection effects (availability of redshifts and reliability of the optical counterpart identification; i.e. Ω(f) from Eq. (1); see Sect. 2.4). The CDFS and COSMOS have nearly complete redshift availability (either spectroscopic of photometric) so selection effects can be ignored.