  We present a combined analysis of the optical spectral variability for two
samples of QSOs, 42 objects at $z<0.4$ monitored at the Wise Observatory
(Giveon et al 1999), plus 59 objects up to $z\sim 3$ in the field of the
Magellanic Clouds, detected and/or monitored within the MACHO Project database
(Geha et al 2003). Our analysis shows some increase of the observed spectral
variability as a function of redshift, with a large scatter. These data are
compared with a model based on the addition of flares of different temperatures
to a stationary quasar SED, taking into account also the intrinsic scatter of
the SEDs.
