  This work aimed to find the relationship between quasars' optical variability
and spectral features to reveal the regularity behind the random variation. It
is known that quasar's FeII/Hbeta flux ratio and equivalent width of [OIII]5007
are negatively correlated, called Eigenvector 1. In this work, we visualized
the relationship between the position on this Eigenvector 1 (EV1) plane and how
they had changed their brightness after ~10 years. We conducted three analyses
using different quasar samples each. The first analysis showed the relation
between their distributions on the EV1 plane and how much they had changed
brightness, using 13,438 Sloan Digital Sky Survey quasars. This result shows
how brightness changes later are clearly related to the position on the EV1
plane. In the second analysis, we plotted the sources reported as
Changing-Look(State) Quasars on the EV1 plane. This result shows that the
position on the EV1 plane corresponds activity level of each source, the bright
or dim state of them are distributed on the opposite sides divided by the
typical quasar distribution. In the third analysis, we examined the transition
vectors on the EV1 plane using sources with multiple-epoch spectra. This result
shows that the brightening and dimming sources move on the similar path and
they turn into the position corresponding to the opposite activity level. We
also found this trend is opposite to the empirical rule that RFeII positively
correlated with the Eddington ratio, which has been proposed based on the
trends of a large number of quasars. From all these analyses, it is indicated
that quasars tend to oscillate between both sides of the distribution ridge on
the EV1 plane; each of them corresponds to a dim state and a bright state. This
trend in optical variation suggests that significant brightness changes, such
as Changing-Look quasars, are expected to repeat.
