def percentile(q, strategy=:midpoint)
          case strategy
          when :midpoint
            midpoint_percentile(q)
          when :linear
            linear_percentile(q)
          else
            raise ArgumentError, "Unknown strategy #{strategy}"
          end
        end
Returns the value of the percentile q

Accepts an optional second argument specifying the strategy to interpolate
when the requested percentile lies between two data points a and b
Valid strategies are:
* :midpoint (Default): (a + b) / 2
* :linear : a + (b - a) * d where d is the decimal part of the index between a and b.
== References

This is the NIST recommended method (http://en.wikipedia.org/wiki/Percentile#NIST_method)