multirow

\begin{table}[htbp]
\caption{Summary of results}
\begin{center}
\begin{adjustbox}{width=\textwidth}
\begin{tabular}{|p{0.40\linewidth}|p{0.1\linewidth}|p{0.1\linewidth}|p{0.1\linewidth}|p{0.1\linewidth}|p{0.1\linewidth}|p{0.1\linewidth}|}

\hline
\multirow{2}{*}{\setstackgap{L}{2.05ex}\Centerstack[l]{\textbf{Measurement}}} &\multicolumn{2}{c|}{\textbf{1 participant}}&\multicolumn{2}{c|}{\textbf{5 participants}}&\multicolumn{2}{c|}{\textbf{9 participants}}\\
\cline{2-7}
&\textbf{Min}&\textbf{Max}&\textbf{Min}&\textbf{Max}&\textbf{Min}&\textbf{Max}\\
\hline
 \textbf{Thoroughness with respect to issue set 1}&39\%&47\%&76\%&92\%&83\%&100\%\\
\hline
\textbf{Thoroughness with respect to issue set 2} &31\%&44\%&74\%&88\%&86\%&100\%\\
\hline
\textbf{Validity}&69\%&86\%&64\%&82\%&58\%&80\%\\
\hline
\textbf{Effectiveness}&28\%&40\%&59\%&74\%&58\%&79\%\\
\hline
\textbf{Reliability}&24\%&36\%&58\%&76\%&-&-\\
\hline

\end{tabular}
\label{sum}
\end{adjustbox}
\end{center}
\end{table}