def std_var
      first_pass = @raw.reduce(0) do |acc, elem|
        (elem.score.to_f - mean)**2 + acc
      end
      Math.sqrt(first_pass / (@raw.size - 1))
    end
def std_var
      first_pass = @raw.reduce(0) do |acc, elem|
        (elem.score.to_f - mean)**2 + acc
      end
      Math.sqrt(first_pass / (@raw.size - 1))
    end
The standard variance (two pass)