Optical variability of quasars: a damped random walk
  A damped random walk is a stochastic process, defined by an exponential
covariance matrix that behaves as a random walk for short time scales and
asymptotically achieves a finite variability amplitude at long time scales.
Over the last few years, it has been demonstrated, mostly but not exclusively
using SDSS data, that a damped random walk model provides a satisfactory
statistical description of observed quasar variability in the optical
wavelength range, for rest-frame timescales from 5 days to 2000 days. The
best-fit characteristic timescale and asymptotic variability amplitude scale
with the luminosity, black hole mass, and rest wavelength, and appear
independent of redshift. In addition to providing insights into the physics of
quasar variability, the best-fit model parameters can be used to efficiently
separate quasars from stars in imaging surveys with adequate long-term
multi-epoch data, such as expected from LSST.
