The long-term optical variability of a composite, complete sample of quasars is investigated. While no correlation with luminosity is detected, a positive correlation with redshift is found (p = 0.25). These results are reconciled with previous opposite findings of other authors, arguing that the analysis of variability can be biased by the combined effect of the structure function of light curves and of cosmological time dilation. Such bias becomes negligible if variability is measured by a magnitude difference at a fixed and large time lag (4 yr). Interpretation of this cosmological trend is given in terms of spectral variability. Statistical biases on the estimate of the evolution rate for the quasar population are discussed.