  The relationship between variability, luminosity and redshift in the South
Galactic Pole QSO sample is examined in an effort to disentangle the effects of
luminosity and redshift in the amplitude of the optical variations. The
anticorrelation between variability and luminosity found by other authors is
confirmed. Our analysis also supports claims that variability increases with
redshift, most likely due to an anticorrelation between variability and
wavelength. In particular, our parametric fits show that the QSO
variability-wavelength relation is consistent with that observed in
low-luminosity nearby active galactic nuclei.
  The results are used to constrain Poissonian-type models. We find that if QSO
variability results from a random superposition of pulses, then the individual
events must have B-band energies between $\sim 10^{50}$ and a few times
$10^{51}$ erg and time-scales of $\sim 2$ yr. Generalized Poissonian models in
which the pulse energy and lifetime scale with luminosity are also discussed.
