As software development techniques have advanced during the last decades, some basic principles of testing have also been established. Describing theoretical ideas and practical hints, these principles can be seen as a basic guideline for both, testing and coding.
In general, testing proves the presence of errors. Sufficient testing reduces the likelihood of existing, not discovered error conditions within the test object. It does not verify that no more bugs exist, even if no more errors can be found. Testing is not a prove that the system is free of errors.
An exhaustive test which considers all possible input parameters, their combinations and different pre-conditions can not be accomplished (except for trivial test objects). Test are always spot tests. Therefore, the effort must be managed by risk, priorities and thoughtful selection.
Testing activities should begin as early as possible within the software life cycle. They should be repeated regularly and have its’ own agenda. Early testing helps detecting errors at an early stage of the development process which simplifies error correction (and reduces the costs for this work).
There is no equal distribution of errors within one test object. The place where one error occurs, it’s likely to find some more. The testing process must be flexible and respond to this behavior.
The effectiveness of tests fades over time. If test-cases are only repeated, they do not expose new errors. Errors, remaining within untested functions may not be discovered. In order to prevent this effect, test-cases must be altered and reworked time by time.
No two systems are the same and therefore can not be tested the same way. Testing intensity, the definition of end criteria etc. must be defined individually for each system depending on its testing context. E-commerce websites require a different approach than online-banking applications.
Error detection and error fixing does not guarantee a usable system matching the users expectations. Early integration of users and rapid prototyping prevents unhappy clients and discussions.