\begin{table}%t3
\caption{Computed $t$-values from the comparison of the contrast for different years with respect to the
activity~minimum contrast (1996, otherwise indicated), for the $B/\mu$ bins shown in Figs.~\ref{fig3}
and \ref{fig4}.} \label{table3} 
%\centerline
{\small\begin{tabular}{llllll} 
\hline 
\hline 
$B/\mu$ & 1997 & 1998 & 1999 & 2000 & 2001 \\
\hline 
$<$ 60 G & 0.05 & 0.21 & 1.18 & 0.40 & 0.21 \\
60 -- 90 G & 0.07 & 0.18 & 1.24 & 0.52 & 0.11 \\
90 -- 130 G & 0.07 & 0.10 & 1.24 & 0.59 & 0.05 \\
130 -- 200 G & 0.03 & 0.22 & 0.99 & 0.52 & 0.10 \\
200 -- 300 G & 0.43 & 1.02 & 0.02 & 0.19 & 0.77 \\
300 -- 400 G$^{{\rm *}}$ & - & 1.01 & 0.30 & 0.27 & 0.81 \\
400 -- 500 G$^{{\rm **}}$ & - & - & 0.12 & 0.20 & 0.30 \\
500 -- 600 G$^{{\rm **}}$ & - & - & 0.63 & 0.40 & 0.09 \\
\hline 
\end{tabular}}\medskip
\par
\small
 $^{\rm*}$~Comparison with respect to 1997; $^{\rm**}$ Comparison with
respect to 1998.
\par
\end{table}