1. 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

2. 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