\documentclass{article}
\usepackage{threeparttable}
\usepackage{multirow}
\usepackage{booktabs}
\begin{document}
\begin{table}[!ht]
\setlength\tabcolsep{0pt}
\centering
\begin{threeparttable}
\begin{tabular}{@{\extracolsep{2ex}}*{4}{lccc}}
\toprule
 &  & \multicolumn{2}{c}{\textbf{group}} \\
\cmidrule{3-4}
 & \textbf{Overall} & 1 & 2 \\
\midrule
\textbf{empty} &  &  &  \\
\hspace{12.0pt}Mean (SD) & 2 (1) & NC & 2 (1) \\
\hspace{12.0pt}Median [Min, Max] & 2 [1, 3] & NC & 2 [1, 3] \\
\hspace{12.0pt}Missing & 3 (50\%) & 3 (100\%) & 0 (0\%) \\
\bottomrule
\end{tabular}
\begin{tablenotes}[flushleft,para]
\footnotesize
NC - Not computable
\end{tablenotes}
\end{threeparttable}
\end{table}
\end{document}