To monitor complex chemical processes, chemical engineers will consider key process indicators, which may be just yield but most often depend on several quantities. Before trying to improve a process, \(n=9\) measurements were made on a key performance indicator.

\[\begin{array}{lllllllll}123 & 106 & 114 & 128 & 113 & 109 & 120 & 102 & 111\end{array}\]

What can the engineer assert with 95% confidence about the maximum error if he uses the sample mean to estimate true mean value of the performance indicator?