  Active galactic nuclei (AGN) that show strong rest-frame optical/UV
variability in their blue continuum and broad line emission are classified as
"changing-look" AGN, or at higher luminosities changing look quasars (CLQs).
These surprisingly large and sometimes rapid transitions challenge accepted
models of quasar physics and duty cycles, offer several new avenues for study
of quasar host galaxies, and open a wider interpretation of the cause of
differences between broad and narrow line AGN. To better characterize extreme
quasar variability, we present follow-up spectroscopy as part of a
comprehensive search for CLQs across the full SDSS footprint using
spectroscopically confirmed quasars from the SDSS DR7 catalog. Our primary
selection requires large-amplitude (|\Delta g|>1 mag, |\Delta r|>0.5 mag)
variability over any of the available time baselines probed by the SDSS and
Pan-STARRS 1 surveys. We employ photometry from the Catalina Sky Survey to
verify variability behavior in CLQ candidates where available, and confirm CLQs
using optical spectroscopy from the William Herschel, MMT, Magellan, and
Palomar telescopes. For our adopted S/N threshold on variability of broad
H\beta emission, we find 17 new CLQs, yielding a confirmation rate of >~ 20%.
These candidates are at lower Eddington ratio relative to the overall quasar
population which supports a disk-wind model for the broad line region. Based on
our sample, the CLQ fraction increases from 10% to roughly half as the
continuum flux ratio between repeat spectra at 3420 Angstroms increases from
1.5 to 6. We release a catalog of over 200 highly variable candidates to
facilitate future CLQ searches.
