Redshift evolution of the correlation length as estimated in different X-ray surveys (COSMOS, Gilli et al. [44], Allevato et al.[45]; CDFN, Gilli et al. [33], Yang et al. [34]; Swift-BAT, Cappelluti et al. [51]; CDFS, Gilli et al. [33]; AEGIS, Coil et al. [48]; AGES, Hickox et al. [47]; ROSAT-NEP, Mullis et al. [24]; ROSAT-SDSS, Krumpe et al. [50]; CLASXS, Yang et al. [34]; RASS, Akylas et al. [22]; ELAIS-S1, Puccetti et al. [32]).
