Testing Principles

Principle 1: 

Testing shows the presence of defects 

Testing can show that defects are present but cannot prove that there are no defects. Testing reduces the probability of undiscovered defects remaining in the software but, even if no defects are found, it is not a proof of correctness.

Principle 2: 

Exhaustive testing

Testing everything (all combinations of inputs and preconditions is impossible ) is not feasible except for trivial cases. Instead of exhaustive testing, we use risks and priorities to focus testing efforts.

Principle 3:

Early testing

Testing activities should start as early as possible in the software or system  development life cycle and should be focused on defined objectives.

Principle 4: 

Defect clustering

A small number of modules contain most of the defects discovered during pre-release testing or show the most operational failures.

Principle 5:

Pesticide paradox

If the same tests are repeated over and 20 over again, eventually the same set of test cases will no longer find any new bugs. To overcome this 'pesticide paradox', the test cases need to be regularly reviewed and revised, and new and different tests need to be written to exercise different parts of the software or system to potentially find more defects.

Principle 6: 

Testing is context dependent

Testing is done differently in different  contexts. For example, safety-critical software is tested differently from an e-commerce site.

Principle 7: 

Absence-of-errors fallacy

Finding and fixing defects does not help if  the system built is unusable and does not fulfill the users' needs and expectations.