class Array
  def mean
    self.inject(0){|res, v| v + res} / self.size
  end
  
  def variance
    sum, sum2 = self.inject([0, 0]){|res, v| 
      [v + res, (v**2) + res]
    }
    return (sum2 - ((sum ** 2) / self.size)) 
  end
end