Order Statistics
The \(k\)-th order statistic of a sample is equal to its \(k\)-th smallest value.
The \(k\)-th order statistic of a set of \(n\) values \(x = \left\{ x_i \right\}, 1 \le i \le n\) is
denoted \(x_{(k)}\). The median of the set \(x\) is equal to \(x_{\left( \frac{n}{2} \right)}\) if
\(n\) is odd, or the average of \(x_{\left( \frac{n}{2} \right)}\) and \(x_{\left( \frac{n}{2} + 1 \right)}\)
if \(n\) is even. The \(k\)-th smallest element of a length \(n\) vector can be found
in average \(O(n)\) time using the quickselect algorithm.
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gsl_stats_select(data, f)
This function finds the k-th smallest element of the input array data.