monkeypatched Array with some statistical methods

array_statistics.rb

class Array
  def mean
    inject(:+).to_f / size
  end

  def median
    s = size
    idx1 = s / 2
    sorted = sort
    if s.odd?
      sorted[idx1]
    else
      idx2 = idx1 - 1
      (sorted[idx1] + sorted[idx2]) / 2
    end
  end

  def mode
    frequencies = group_by { |i| count i}
    frequencies[frequencies.keys.max].first
  end

  def variance
    s = size
    sum, sum_of_squares = 0.0, 0.0

    each do |item|
      sum += item
      sum_of_squares += item * item
    end

    sum_of_squares / s - sum * sum / s / s
  end

  def std_deviation
    variance ** 0.5
  end

  # Magic number of 1.96 gives us 95% confidence:
  MAGIC_NUMBER = 1.96

  def confidence_interval(magic_number = MAGIC_NUMBER)
    magic_number * (variance / size) ** 0.5
  end

  def confidence_boundaries(magic_number = MAGIC_NUMBER)
    m = mean
    ci = confidence_interval(magic_number)
    [m - ci, m + ci]
  end
end