Consider data collected into disjoint intervals of the form
\(\displaystyle [a_1,b_1)\)
\(\displaystyle [a_2,b_2)\)
\(\displaystyle [a_3,b_3)\)
...
\(\displaystyle [a_n,b_n)\)
where \(f_k\) is the frequency of data items in interval \([a_k,b_k)\text{.}\) Generally, since the intervals are disjoint then let’s put them in order from low to high so that \(b_1 \le a_2, b_2 \le a_3\text{,}\) etc. Compute class marks \(mid_k = \frac{a_k+b_k}{2}\) and then use
\begin{equation*}
\mu = \frac{mid_1 f_1 + ... + mid_m f_m}{f_1 + ... + f_m} = \frac{\sum_{k=1}^m mid_k f_k}{\sum_{k=1}^m f_k}.
\end{equation*}