Definition 4.2.1. Cumulative relative frequency.
For a collection of ordered events \(x_1 \lt x_2 \lt ... \lt x_s\) with corresponding frequencies \(f_1, f_2, ..., f_s\text{,}\) the cumulative relative frequency is the function
\begin{equation*}
F(x) = \sum_{x_k \le x} f_{x_k}
\end{equation*}