The Utility and Practicality of Quantifying Software Reliability
  We argue that quantifying software reliability is important in demonstrating that system-level risks are As Low As Reasonably Practicable (ALARP). Furthermore, we demonstrate that such quantification is possible in at least one meaningful case. It is, however, unlikely to be practical in every case. This means it is unlikely to be included as an explicit objective in standards. Hence, for those cases where software reliability can be quantified, merely following a standard may lead to risk-reduction opportunities being missed. 
Rob Ashmore
cs.SE