code: def accuracy(self):
          r
          if self.population() == 0:
              return float('NaN')
          return (self._tp + self._tn) / self.population()
docs: r"""Return accuracy.
  
          Accuracy is defined as :math:`\frac{tp + tn}{population}`
  
          Cf. https://en.wikipedia.org/wiki/Accuracy
  
          Returns
          -------
          float
              The accuracy of the confusion table
  
          Example
          -------
          >>> ct = ConfusionTable(120, 60, 20, 30)
          >>> ct.accuracy()
          0.782608695652174