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 .