evolution. These two opposite models predict a significantly different redshift evolution of the bias factor [@mclm97; @mclm98].

The first attempt to measure the clustering of QSOs was made by @osmer81. @s84 was the first to detect QSO clustering on small scales using the @VV84 catalog, a collection of inhomogeneous samples. A number of authors [@is88; @ac92; @mf93; @sb94; @andreani; @cs96] have used complete and better defined QSO samples to measure their spatial distribution. At a mean redshift of $z\sim 1.4$, they generally detect a clustering signal at a typical significance level of $\sim3-4\sigma$, corresponding to a correlation length, $r_0$, similar to the value obtained for local galaxies: $r_0\sim6 h^{-1}$ Mpc. However, there has been significant disagreement over the redshift evolution of QSO clustering, including claims for a decrease of $r_0$ with redshift [@is88], an increase of $r_0$ with redshift [@lac98] and no change with redshift [@cs96]. Recently, @2qzclust, using more than 10,000 objects taken from the preliminary data release catalog of 2dF QSO Redshift Survey (hereafter 2QZ), measured the evolution of QSO clustering as a function of redshift. Assuming an Einstein-de Sitter universe ($\Omega_{M}=1.0$ and $\Omega_{\Lambda}=0.0$), they found no significant evolution for $r_0$ in comoving coordinates over the redshift range 