We perform a systematic search for high-redshift ( z > 1.5 ) extreme variability quasars ( EVQs ) using repeat spectra from the Sixteenth Data Release of Sloan Digital Sky Survey , which provides a baseline spanning up to \sim 18 yrs in the observed frame .
We compile a sample of 348 EVQs with a maximum continuum variability at rest frame 1450 Å of more than 100 % ( i.e. , \delta V \equiv ( Max - Min ) /Mean > 1 ) .
Among those EVQs , we discover 23 new Changing-Look Active Galactic Nuclei ( CLAGNs ) with CL behaviors in different UV lines , e.g. , C iv , C iii ] , Si iv and Mg ii .
Importantly , spurious CLAGNs caused by SDSS problematic spectral flux calibration , e.g. , fiber drop issue , are rejected .
Compared with normal EVQs , we find that CLAGNs have systematically lower continuum luminosity and broader emission line widths , hence lower Eddington ratios .
Statistical tests of EVQ/CLAGN properties suggest that CLAGNs are likely to be a subset of EVQs with less efficient accretion rather than a distinctive AGN population .
Moreover , we find that the difference composite spectra ( i.e. , bright - faint ) of EVQs and CLAGNs are both similar to those of normal quasars with a power-law slope of \alpha _ { \lambda,diff } \sim - 2 , further supporting that they belong to the same population with analogous intrinsic variability mechanisms , e.g. , rapid accretion rate change with a possible state transition .
Finally , we also confirm the anti-breathing of C iv ( i.e. , line width increases as luminosity increases ) in EVQs , and find that in addition to \sim 0.4 dex systematic uncertainty in single-epoch C iv virial black hole mass estimates , an extra scatter of \sim 0.3 dex will be introduced by extreme variability .