How can I do standard deviation in Ruby?

Question

I have several records with a given attribute, and I want to find the standard deviation.

How do I do that?

Answer 1

The answer given above is elegant but has a slight error in it. Not being a stats head myself I sat up and read in detail a number of websites and found this one gave the most comprehensible explanation of how to derive a standard deviation. http://sonia.hubpages.com/hub/stddev

The error in the answer above is in the sample_variance method.

Here is my corrected version, along with a simple unit test that shows it works.

in ./lib/enumerable/standard_deviation.rb

#!usr/bin/ruby

module Enumerable

  def sum
    return self.inject(0){|accum, i| accum + i }
  end

  def mean
    return self.sum / self.length.to_f
  end

  def sample_variance
    m = self.mean
    sum = self.inject(0){|accum, i| accum + (i - m) ** 2 }
    return sum / (self.length - 1).to_f
  end

  def standard_deviation
    return Math.sqrt(self.sample_variance)
  end

end

in ./test using numbers derived from a simple spreadsheet.

#!usr/bin/ruby

require 'enumerable/standard_deviation'

class StandardDeviationTest < Test::Unit::TestCase

  THE_NUMBERS = [1, 2, 2.2, 2.3, 4, 5]

  def test_sum
    expected = 16.5
    result = THE_NUMBERS.sum
    assert result == expected, "expected #{expected} but got #{result}"
  end

  def test_mean
    expected = 2.75
    result = THE_NUMBERS.mean
    assert result == expected, "expected #{expected} but got #{result}"
  end

  def test_sample_variance
    expected = 2.151
    result = THE_NUMBERS.sample_variance
    assert result == expected, "expected #{expected} but got #{result}"
  end

  def test_standard_deviation
    expected = 1.4666287874
    result = THE_NUMBERS.standard_deviation
    assert result.round(10) == expected, "expected #{expected} but got #{result}"
  end

end