Issues in Safety Assurance

Abstract

The greatest problem facing the developer of a software based safety-related system is the challenge of showing that the system will provide the required service and will not cause or allow an accident to occur. It is very difficult to provide such evidence before the system is put to use, yet that is exactly what is required by society and regulators, and rightly so. Conventional wisdom recommends that systems are classified into safety integrity levels (SILs) based on some combination of the allowable rate or probability of unsafe failure and the probable consequences of such a failure; then, depending on the SIL, development methods are chosen that will (it is hoped) deliver the necessary system quality and the evidence on which to base a confident assessment that the system is, indeed, safe enough. Such conventional wisdom is founded on a number of unstated axioms, but computing is a young discipline and progress has thrown doubt on these assumptions. It is time for a new approach to safety assurance.