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\begin{document}
\begin{tabular}{lcccc}
\toprule
\multirow{2}{*}{Parameter} & \multirow{2}{*}{Value} & \multicolumn{3}{c}{Covariance} \\
\cmidrule{3-5}
          &       & $a^{\rm BCL}_0$ & $a^{\rm BCL}_1$ & $a^{\rm BCL}_2$ \\
\midrule
$a^{\rm BCL}_0$ & $\phantom{-}0.66574$ & $0.00015$ & $0.00022$ & $0.00003$ \\
$a^{\rm BCL}_1$ & $-3.23599$ &           & $0.20443$ & $0.10080$ \\ 
$a^{\rm BCL}_2$ & $-0.07478$ &           &           & $4.04413$ \\ 
\bottomrule
\end{tabular}
\end{document}
