code: def population(self):
          
          return self._tp + self._tn + self._fp + self._fn
docs: Return population, N.
  
          Returns
          -------
          int
              The population (N) of the confusion table
  
          Example
          -------
          >>> ct = ConfusionTable(120, 60, 20, 30)
          >>> ct.population()
          230