**Table 1:** Simulation of the effect of completeness. We have simulated two distributions of sources which obey the linear relationship given in Eq. (2). We have then excluded a part of this sample and looked for a relationship. The power law index is indicated in this table together with its error (errors are quoted at the 1 $\sigma$ level).
$\begin{displaymath}\begin{tabular}{ccc} \hline\hline \noalign{\smallskip } Sa... ...92 $\pm$ 0.12\\ \noalign{\smallskip } \hline \end{tabular} \end{displaymath}$