Descriptive statistics in Ruby

descriptive_statistics.rb

# Add methods to Enumerable, which makes them available to Array
module Enumerable

  # sum of an array of numbers
  def sum
    return self.inject(0,:+)
  end

  # average of an array of numbers
  def mean
    return self.sum / self.size.to_f
  end

  # biased sample variance of an array of numbers
  def biased_sample_variance
    mean = self.mean
    return self.map{ |sample| (mean - sample) ** 2 }.inject(0,:+) / self.size.to_f
  end

  # unbiased sample variance of an array of numbers
  def unbiased_sample_variance
    mean = self.mean
    return self.map{ |sample| (mean - sample) ** 2 }.inject(0,:+) / (self.size - 1).to_f
  end

  # standard deviation of the sample of an array of numbers
  def standard_deviation
    return Math.sqrt(self.biased_sample_variance)
  end

  # sample standard deviation of an array of numbers
  def sample_standard_deviation
    return Math.sqrt(self.unbiased_sample_variance)
  end

  # standard error of an arrya of numbers
  def standard_error
    return self.sample_standard_deviation / Math.sqrt(self.size).to_f
  end

end # module Enumerable