Quasar Variability Measurements With SDSS Repeated Imaging and POSS Data
  We analyze the properties of quasar variability using repeated SDSS imaging data in five UV-to-far red photometric bands, accurate to 0.02 mag, for 13,000 spectroscopically confirmed quasars. The observed time lags span the range from 3 hours to over 3 years, and constrain the quasar variability for rest-frame time lags of up to two years, and at rest-frame wavelengths from 1000 Ang. to 6000 Ang. We demonstrate that 66,000 SDSS measurements of magnitude differences can be described within the measurement noise by a simple function of only three free parameters. The addition of POSS data constrains the long-term behavior of quasar variability and provides evidence for a turn-over in the structure function. This turn-over indicates that the characteristic time scale for optical variability of quasars is of the order 1 year. 
Z Ivezic, R.H. Lupton, M. Juric, S. Anderson, P.B. Hall, G.T.   Richards, C.M. Rockosi, D.E. Vanden Berk, E.L. Turner, G.R. ...