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 $zsim 3$ 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.
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