The current Percentile implementation assumes and hard-codes the quantile pth position as
p * (N+1)/100 and provides a kth selected value.
However if we need to verify compare/contrast with standard statistical tools such as say MS Excel; it would be good to provide an extensible way of morphing this selection of position than hard code.
For example in order to generate the percentile closely matching with MS Excel the position required may be [p*(N-1)/100]+1.
Please let me know if i could submit this as a patch.