No matter how hard you try, you will never get an error free system! Defects remaining in code Current chaos theory Reality as demonstrated by Myers (1979) Key Learning Points: Testing is about risk management. Must focus on aspects of the systems which could cause the most problems if they failed to operate correctly. For example the system supporting the scheduling of vehicles for the London Ambulance Service would have a higher risk than a system supporting staff training for the same organisation. The traditional view Testing effort

Space Shuttle on-board flight software Errors observed Space Shuttle on-board flight software 60 50 40 30 20 10 1988 1989 1990 1991 1992 1993 Source: Ted Keller, manager of Space Shuttle Project Co-ordination for Loral Space Information Systems Key Learning Point: * this graph does not include the shuttle that blew up - that was not a software error but a faulty piece of kit. Total released errors Total failures Failures in flight