comparison with the linear theory of the structure evolution. Since in the following sections we will compare our results with those obtained for the 2QZ by @2qzclust, we prefer to quote clustering amplitudes within $20 h^{-1}$ Mpc, a scale for which linearity is expected to better than a few per cent. Choosing a large radius also reduces the effects of small scale peculiar velocities and redshift measurement errors, which may well be a function of redshift.

Table \[tabclust\]  summarizes the values of $r_0$, $\gamma$ and $\bar{\xi}(20)$ for the total sample, both for the EdS and $\Lambda$ models. In the same table, we list the mean redshift ($\bar{z}$) of the observed QSO sample. We also report the median value of the redshift of the QSO pairs computed within a sphere of $20 h^{-1}$ Mpc, $z_{\xi}$. We find that it is systematically lower than the mean redshift of the sample.

In our analysis, we do not take into account the velocity field of QSOs, the cone edge effect and the effect of statistical errors on QSO redshifts. Recent papers \[see e.g. @2qzclust\] suggest that the Poisson errors, due to the limited size of a sample, are more important than these effects.

Comparison 