  The long-term variability of a sample of 149 optically selected QSOs in the
field of the Selected Area 94 has been studied in the R-band. The relations
between variability and luminosity and between variability and redshift have
been investigated by means of ``robust'' statistical estimators, allowing to
disentangle the effects of the measurement errors. The results are compared
with the corresponding properties of the variability in the B-band for the same
sample. An anti-correlation between the R-band variability and the intrinsic
luminosity is found, analogously to what is observed in the B-band. The
amplitude of the R-band variability turns out to be smaller (of a factor $1.13
\pm 0.05$) than the B-band variability. The implications in terms of the
black-hole, starburst and microlensing models are discussed.
