that this procedure is equivalent to measure the real-space halo-halo correlation function for haloes of different masses (Sheth, Mo & Tormen, 2001, Padilla & Baugh, 2002), with the advantage of avoiding large uncertainties from small samples of haloes restricted to narrow ranges of halo mass in the simulation. We show one example of this procedure in Figure \[fig:xir\], where we find the mass of haloes with an equivalent clustering amplitude to the sample of red galaxies in the SDSS-DR6 (O2). In this figure, the top panel shows the auto-correlation function, $\xi(\sigma,\pi)$, of O2 galaxies, and the lower panel the resulting real-space correlation function (symbols) and the best fit $\Lambda$CDM curve. From this analysis we find that red galaxies in the SDSS-DR6 (with $b/a<0.7$) are consistent with haloes of a median mass of $\log_{10}(M/h^{-1}M_{\odot})=13.06^{+0.19}_{-0.27}$. This provides us with a quantitative way to compare the SDSS-DR6 subsamples to the halo samples in the numerical simulations. Also, in order to infer this scale for our observational samples, we will use the dependence of the 1- to 2-halo term transition on DM halo mass measured in our the numerical simulations.

The other SDSS-DR6 galaxy samples are fitted by the correlation function of DM haloes 