code: def delta(self):
          
  
          v = float(self._logistic.value)
  
          if v > 0.0:
              v = 1 / (1 + exp(-v))
          else:
              v = exp(v)
              v = v / (v + 1.0)
  
          return min(max(v, epsilon.tiny), 1 - epsilon.tiny)
docs: Variance ratio between ``K`` and ``I``.