What does software safety integrity mean in the context of safety integrity being defined as probability of failure?
A safety integrity level (SIL) applies to an end-to-end safety function of the safety-related system. Like any other system component, software has no safety integrity level in isolation from the safety-related system. When integrated into a system, software may be capable of supporting a particular safety function at some safety integrity level, depending on how the software was specified, designed, implemented, verified, etc. SILn software is a short way of saying ‘software developed using appropriate techniques and measures to ensure that the software meets the systematic failure requirements of a specific safety function X at SILn’. Hardware suffers physical degradation and the resulting random failure rates can be described numerically using well established methods of statistical reliability. In contrast, software does not degrade physically, and all failures result from systematic factors in its construction and use. It is not currently widely accepted that conventional reliabil
