Recall that the percentile is the value such that at least interest-percent of the data set items are less than or equal to this value, or equivalently if at least (100-interest) percent of the items are greater than or equal to this value.

Remarks:

1. The 50-th percentile is the measure of centrality of a data set known as the median. The median can also be evaluated using FinancialMedian.
2. The 25-th, 50-th, 75-th percentiles are often referred to as the 1-st, 2-nd (median) and 3-rd quartile respectively. The term quartile refers to the fact that these three values will roughly divide the data set considered into four equal parts.

Reference

StatisticalEngine Class
StatisticalEngine Members
The first percentile.
The third percentile.
Percentile50(Series) Method