  Significant progress in the description of quasar variability has been
recently made by employing SDSS and POSS data. Common to most studies is a
fundamental assumption that photometric observations at two epochs for a large
number of quasars will reveal the same statistical properties as well-sampled
light curves for individual objects. We critically test this assumption using
light curves for a sample of $\sim$2,600 spectroscopically confirmed quasars
observed about 50 times on average over 8 years by the SDSS stripe 82 survey.
We find that the dependence of the mean structure function computed for
individual quasars on luminosity, rest-frame wavelength and time is
qualitatively and quantitatively similar to the behavior of the structure
function derived from two-epoch observations of a much larger sample. We also
reproduce the result that the variability properties of radio and X-ray
selected subsamples are different. However, the scatter of the variability
structure function for fixed values of luminosity, rest-frame wavelength and
time is similar to the scatter induced by the variance of these quantities in
the analyzed sample. Hence, our results suggest that, although the statistical
properties of quasar variability inferred using two-epoch data capture some
underlying physics, there is significant additional information that can be
extracted from well-sampled light curves for individual objects.
