Software Systems While fault tolerant software is seen as a necessity, it is also controversial and a factor of regulatory uncer-tainty. Up to date researchers do not know what creditable reliability models for fault tolerant software are, how to test for fault tolerance, and how effective fault tolerant software can achieve. In particular, we cannot system-atically develop models to predict reliability of fault tolerant software systems, and provide evidences regarding the validity of these models. One difficulty lies on the fact that there is no proper model to describe the nature and interactions of software faults regarding how they are manifested and how they are correlated. As currently, there are no quantitative assessment schemes fo...