def variance(sample = false)
      a = numerify
      avg = a.average
      sum = a.inject(0) { |sum, value| sum + (value - avg) ** 2}
      (1 / (a.length.to_f - (sample ? 1 : 0)) * sum)
    end
def variance(sample = false)
      a = numerify
      avg = a.average
      sum = a.inject(0) { |sum, value| sum + (value - avg) ** 2}
      (1 / (a.length.to_f - (sample ? 1 : 0)) * sum)
    end
variance of an array of numbers