  The issue of the X-ray to optical luminosity relationship ($L_o-L_x$) is
addressed for both optically and X-ray selected quasar samples. We have applied
a generalized regression algorithm for the case of samples involving censored
data, with errors on both the dependent and the independent variable. Contrary
to some previous results, we find that such relationship is consistent with
being a linear one ($L_x\propto L_o$). We argue that previous reports of
non-linear relationships (i.e. $L_x\propto L_o^e$ with $e < 1$) are due to the
neglect of the influence of the photometric errors, the precise knowledge of
which strongly influences the reliability of the results. Further progresses in
the determination of the $L_o-L_x$ relationship can be achieved with ROSAT
observations of the new generation of large bright quasar surveys.
