def variance
      μ = mean
      reduce(0) { |m, i| m + (i - μ).square } / (length - 1)
    end
def variance
      μ = mean
      reduce(0) { |m, i| m + (i - μ).square } / (length - 1)
    end
Returns the variance of an Array of Numerics.

   [1, 2, 3, 4, 5].variance  #=> 2.5
   [1.0, 2.0, 3.0].variance  #=> 1.0