Theorem 1.3.13.
Given any value y with \(y_1 < y < y_n\text{,}\) then y is at the \(P^s\) percentile with
\begin{equation*}
s = \frac{r+m}{n+1}
\end{equation*}
where n = the number of data items, m = the positional subscript such that \(y_m < y < y_{m+1}\) and \(r = \frac{y - y_m}{y_{m+1}} - y\)