In the aircraft industry a figure of 10 -9 accidents per hour is often quoted. Further reliability could be added with software verification and validation, but measurements on the effectiveness of such approaches are hard to come by. In any case, relying solely on software seems to be a foolhardy approach given these figures. Despite this, it is sometimes assumed that software is totally reliable in some calculations since software does not wear out in the same way that hardware does. Instead, the errors occur in a much more random and unpredictable manner.
Software is a digital rather than analogue artifact.
As such, techniques such as interpolation and extrapolation, used by many hardware engineers in calculations, do not apply. Changing a single bit in a computer program can have a very unpredictable effect on its operation, or may be completely benign in some circumstances. Given the above problems with the inclusion of software in safety-critical systems, the decision to depend on software to ensure the safety of a system should be taken with great caution. Hardware backup is normally recommended where possible.