Testing Reveals the Presence of Defects: Testing can uncover defects in software, but it cannot guarantee that there are no defects. Even with thorough testing, it’s impossible to claim that the product is entirely free of issues. Testing helps reduce the number of undiscovered defects, but a lack of detected defects does not prove the software’s correctness.

Exhaustive Testing is Impractical: It’s impossible to test every possible combination of inputs and conditions. Instead of attempting exhaustive testing, it’s more effective to prioritize and focus on high-risk areas. For example, if a screen has 15 input fields with 5 possible values each, the number of potential test cases is astronomical, making exhaustive testing unrealistic within typical project timelines. Prioritizing and managing risk is crucial.

Start Testing Early: Testing should begin as early as possible in the software development life cycle, with a clear focus on defined objectives. Early testing helps identify and address defects sooner, reducing the risk of costly fixes later on.

Defect Clustering: A small number of modules often contain the majority of defects identified during pre-release testing or are responsible for most operational failures. This principle highlights the importance of targeted testing in these high-risk areas.

The Pesticide Paradox: Repeating the same tests eventually leads to diminishing returns, as these tests become less effective at finding new defects. To counter this, it’s vital to regularly review and update test cases, introducing new tests to explore different aspects of the software.

Testing is Context-Dependent: Testing approaches vary depending on the context. Different types of software require different testing strategies. For example, safety-critical systems are tested with much stricter protocols than a standard e-commerce site.

The Absence-of-Errors Fallacy: Even if a system is free of defects, it doesn’t mean it’s successful. If the software doesn’t meet user needs and expectations, the absence of errors is irrelevant. Testing must ensure that the software is both functional and usable.