A number of testing principles have been suggested over the past 40 years and offer general guidelines common for all testing.
Principle 1 – Testing shows 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 is impossible
Testing everything (all combinations of inputs and preconditions) is not feasible except for trivial cases. Instead of exhaustive testing, risk analysis and priorities should be used 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 are responsible for the most operational failures.
Principle 5 – Pesticide paradox
If the same tests are repeated over and over again, eventually the same set of test cases will no longer find any new defects. 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.