The X-ray variability of active galactic nuclei (AGN) has been most often investigated with studies of individual, nearby sources, and only a few ensemble analyses have been applied to large samples in wide ranges of luminosity and redshift. In their study, the authors aimed to determine the ensemble variability properties of two serendipitously selected AGN samples extracted from the catalogs of XMM-Newton and Swift (the latter is not included in this table, notice), with redshift between ~ 0.2 and ~ 4.5, and X-ray luminosities, in the 0.5 - 4.5 keV band, between ~ 10⁴³ erg/s and ~ 10⁴⁶ erg/s. They used the structure function (SF), which operates in the time domain, and allows for an ensemble analysis even when only a few observations are available for individual sources and the power spectral density (PSD) cannot be derived. The SF is also more appropriate than fractional variability and excess variance, because these parameters are biased by the duration of the monitoring time interval in the rest-frame, and therefore by cosmological time dilation. The authors find statistically consistent results for the two samples, with the SF described by a power law of the time lag tau, approximately as SF ~ tau^0.1. They do not find evidence of the break in the SF, at variance with the case of lower luminosity AGNs. They confirm a strong anti-correlation of the variability with X-ray luminosity, accompanied by a change of the slope of the SF. They also find evidence in support of a weak, intrinsic, average increase of X-ray variability with redshift. For XMM, the authors used the version of the Serendipitous Source Catalog then available, namely 2XMMi-DR3, the latest incremental update of the second version of the catalogue, with observations made between 2000 February 3 and 2008 October 08; all datasets were publicly available by 2009 October 31, but not all public observations are included in this catalog. The total area of the catalog fields is ~ 814 deg², but taking account of the substantial overlaps between observations, the net sky area covered independently is ~ 504 deg². The 2XMMi-DR3 catalogue contains 353,191 detections (above the processing likelihood threshold of 6), related to 262,902 unique X-ray sources, therefore a significant number of sources (41,979) have more than one record within the catalog. The selected sources were cross-correlated with the DR7 edition of the SDSS Quasar Catalog (Schneider et al. 2010, AJ, 139, 2360) to obtain redshifts and spectral classifications for the sources. The authors used a maximum distance of 1.5 arcseconds, corresponding to the uncertainty in the X-ray positions, resulting in 412 quasars that were observed by XMM-Newton from 2 to 25 epochs each for a total of 1376 observations. The authors refer to these sources as the XMM-Newton sample. This table was created by the HEASARC in April 2012 based on <a href="https://cdsarc.cds.unistra.fr/ftp/cats/J/A+A/536/A84">CDS Catalog J/A+A/536/A84</a> file table1.dat. This is a service provided by NASA HEASARC .