Calculate 95th percentile in Ruby?

Question

This question here does not seem to help: Calculating Percentiles (Ruby)

I would like to calculate 95th percentile (or, indeed, any other desired percentile) from an array of numbers. Ultimately, this will be applied in Rails to calculate distribution against a large number of records.

But, if I can determine how to accurately determine a given percentile from an array of numbers, I can take it from there.

Frankly, I am surprised that I haven't been able to find some sort of gem that would have such functions--I haven't found one yet.

Help is greatly appreciated.

Answer 1

If you want to replicate Excel's PERCENTILE function then try the following:

def percentile(values, percentile)     values_sorted = values.sort     k = (percentile*(values_sorted.length-1)+1).floor - 1     f = (percentile*(values_sorted.length-1)+1).modulo(1)          return values_sorted[k] + (f * (values_sorted[k+1] - values_sorted[k])) end  values = [1, 2, 3, 4] p = 0.95 puts percentile(values, p) #=> 3.85 

The formula is based on the QUARTILE method, which is really just a specific percentiles - https://support.microsoft.com/en-us/office/quartile-inc-function-1bbacc80-5075-42f1-aed6-47d735c4819d.