Calculating Percentiles (Ruby).

Posted by zxcvbnm on Stack Overflow See other posts from Stack Overflow or by zxcvbnm
Published on 2010-04-09T17:39:38Z Indexed on 2010/04/09 17:43 UTC
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My code is based on the methods described here and here.

def fraction?(number)
  number - number.truncate
end

def percentile(param_array, percentage)
  another_array = param_array.to_a.sort
  r = percentage.to_f * (param_array.size.to_f - 1) + 1
  if r <= 1 then return another_array[0]
  elsif r >= another_array.size then return another_array[another_array.size - 1]
  end
  ir = r.truncate
  another_array[ir] + fraction?((another_array[ir].to_f - another_array[ir - 1].to_f).abs)
end

Example usage:

test_array = [95.1772, 95.1567, 95.1937, 95.1959, 95.1442, 95.061, 95.1591, 95.1195,
95.1065, 95.0925, 95.199, 95.1682]
test_values = [0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0]

test_values.each do |value|
  puts value.to_s + ": " + percentile(test_array, value).to_s
end

Output:

0.0: 95.061
0.1: 95.1205
0.2: 95.1325
0.3: 95.1689
0.4: 95.1692
0.5: 95.1615
0.6: 95.1773
0.7: 95.1862
0.8: 95.2102
0.9: 95.1981
1.0: 95.199

The problem here is that the 80th percentile is higher than the 90th and the 100th. However, as far as I can tell my implementation is as described, and it returns the right answer for the example given (0.9).

Is there an error in my code I'm not seeing? Or is there a better way to do this?

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