\begin{table}%t2 
\caption {\label{table_stat_highpurity}Quality statistics ($C_1$, $C_2$, $P_1$, $P_2$,
$S_{\rm gal}$, and $f_{\rm I}$, see text for details) of the group catalogue 
reconstructed by the algorithm with the {\it high-purity} parameter
set, for two different redshift bins and the whole redshift
range, considering groups with $N\geq 2$. }
%\centering
\par
\begin{tabular}{c c c c}
 \multicolumn{4}{c}{Quality statistics for $N\geq 2$}\\  
\hline\hline 
Quality parameter & $0.2\leq z \leq 0.6$ & $0.6\leq z \leq 1.0$ &  $0.2\leq z \leq 1.0$\\    
\hline                        
   $C_1$          & $0.29 \pm 0.03$      & $0.20 \pm 0.03$      &  $0.24 \pm 0.02$     \\
   $C_2$          & $0.24 \pm 0.03$      & $0.17 \pm 0.02$      &  $0.20 \pm 0.02$     \\
   $P_1$          & $0.75 \pm 0.04$      & $0.73 \pm 0.06$      &  $0.74 \pm 0.04$     \\
   $P_2$          & $0.66 \pm 0.07$      & $0.69 \pm 0.06$      &  $0.67 \pm 0.05$      \\
   $S_{\rm gal}$      & $0.32 \pm 0.03$      & $0.21 \pm 0.04$      &  $0.28 \pm 0.03$        \\
   $f_{\rm I}$        & $0.24 \pm 0.04$      & $0.27 \pm 0.05$      &  $0.25 \pm 0.03$        \\
\hline                                   
\end{tabular}
\tablefoot {Each parameter is computed 
as the mean over the 20~VVDS-like
mocks, and the associated error is its $rms$. The same parameters for 
groups with $N\geq 3$ are consistent, within error bars,
with those presented here.}
\end{table}