code: def fdr(self):
          r
          if self._fp + self._tp == 0:
              return float('NaN')
          return self._fp / (self._fp + self._tp)
docs: r"""Return false discovery rate (FDR).
  
          False discovery rate is defined as :math:`\frac{fp}{fp + tp}`
  
          Cf. https://en.wikipedia.org/wiki/False_discovery_rate
  
          Returns
          -------
          float
              The false discovery rate of the confusion table
  
          Example
          -------
          >>> ct = ConfusionTable(120, 60, 20, 30)
          >>> ct.fdr()
          0.14285714285714285