The time estimates obtained from four contractors (P, Q, R and S) for executing a particular job are as under:
\[\begin{array}{|c|c|c|c|} \hline \textbf{Contractor} & \textbf{Optimistic time, $t_o$} & \textbf{Most likely time, $t_m$} & \textbf{Pessimistic time, $t_p$} \\ \hline \text{P} & 5 & 10 & 13 \\ \hline \text{Q} & 6 & 9 & 12 \\ \hline \text{R} & 5 & 10 & 14 \\ \hline \text{S} & 4 & 10 & 13 \\ \hline \end{array}\]
Which of these contractors is more certain about completing the job in time?