  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 Angstrom of more than 100% (i.e.,
$\delta$V $\equiv$ (Max$-$Min)/Mean $>$1). The EVQs show a range of emission
line variability, including 23 where at least one line in our redshift range
disappears below detectability, which can then be seen as analogous to
low-redshift changing-look quasars (CLQs)". Importantly, spurious CLQs caused
by SDSS problematic spectral flux calibration, e.g., fiber drop issue, have
been rejected. The similar properties (e.g., continuum/line,
difference-composite spectra and Eddington ratio) of normal EVQs and CLQs,
implies that they are basically the same physical population with analogous
intrinsic variability mechanisms, as a tail of a continuous distribution of
normal quasar properties. In addition, we find no reliable evidence ($\lesssim$
1$\sigma$) to support that the CLQs are a subset of EVQs with less efficient
accretion. 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.

Astrophysics of Galaxies