3. Test selection

It is important to realise that with the best will in the world it is not possible to test every part of the system.

Consider a system having 8 data inputs that can applied in any combination. The maths says that there are over 16 million possible combinations of just 8 inputs!

So the skill of the testing team is to carefully select a practical number of tests that can be carried out in the time available and yet be confident that the system is working.

This is why complicated software often has bugs even after being released. It is simply not practical to test everything.