completely random distribution, the expected number of pairs is 12 for EdS and 14 for the $\Lambda$ model. Considering separations smaller than $20   
h^{-1}$ Mpc the observed pairs are 36 and 38, to be compared with 27 and 26 random pairs expected. The clustering signal is therefore detected at a $3-4\sigma$ level.

The differential TPCF for the complete AERQS sample is shown in Fig. \[totlog\], both for EdS (upper panel) and $\Lambda$ models (lower panel). The results have been fitted by adopting a power-law relation $$\xi(r)=\left(\frac{r}{r_{0}}\right)^{-\gamma}\ .   
\label{eq:powlaw}$$

The best-fit parameters can be obtained by using a maximum likelihood estimator (MLE) based on Poisson statistics and unbinned data [@croft]. Unlike the usual $\chi^2$-minimization, this method allows to avoid the uncertainties due to the bin size (see above), the position of the bin centers and the bin scale (linear or logarithmic).

To build the estimator, it is necessary to estimate the predicted probability distribution of quasar pairs, given a choice for the correlation length $r_0$ and the slope $\gamma$. The small number of pairs observed at small scales makes a reliable determination of the slope $\gamma$ particularly difficult. Therefore, we have used fixed values for the slope $\gamma$, adopting those obtained 