code: def accuracy_gain(self):
          r
          if self.population() == 0:
              return float('NaN')
          random_accuracy = (self.cond_pos_pop() / self.population()) ** 2 + (
              self.cond_neg_pop() / self.population()
          ) ** 2
          return self.accuracy() / random_accuracy
docs: r"""Return gain in accuracy.
  
          The gain in accuracy is defined as:
          :math:`G(accuracy) = \frac{accuracy}{random~ accuracy}`
  
          Cf. https://en.wikipedia.org/wiki/Gain_(information_retrieval)
  
          Returns
          -------
          float
              The gain in accuracy of the confusion table
  
          Example
          -------
          >>> ct = ConfusionTable(120, 60, 20, 30)
          >>> ct.accuracy_gain()
          1.4325259515570934