Why Software testing is necessary?

An error is a human action that produces an incorrect result. A fault is a manifestation of an error in software (also known as a defect or bug). A fault, if encountered, may cause a failure, which is a deviation of the software from its expected delivery or service. Reliability is the probability that software will not cause the failure of a system for a specified time under specified conditions. Errors occur because we are not perfect and, even if we were, we are working under constraints such as delivery deadlines.

Testing identifies faults, whose removal increases the software quality by increasing the software’s potential reliability. Testing is the measurement of software quality. We measure how closely we have achieved quality by testing the relevant factors such as correctness, reliability, usability, maintainability, reusability, testability, etc.

Other factors that may determine the testing performed may be contractual requirements, or legal requirements, normally defined in industry-specific standards, or based on agreed best practice (or more realistically non negligent practice).

Though It is very difficult to determine how much testing is enough because sometimes A single failure can cost nothing or a lot. Software in safety-critical systems can cause death or injury if it fails, so the cost of a failure in such a system may be in human lives.

The amount of testing performed depends on the risks involved. Risk must be used as the basis for allocating the test time that is available and for selecting what to test and where to place emphasis.