Every software product has a target audience. For example, the audience for video game software is completely different from banking software. Therefore, when an organization develops or otherwise invests in a software product, it can assess whether the software product will be acceptable to its end users, its target audience, its purchasers, and other stakeholders. Software testing assists in making this assessment. Faults and failures Software faults occur through the following process: A programmer makes an error (mistake), which results in a fault (defect, bug) in the software source code. If this fault is executed, in certain situations the system will produce wrong results, causing a failure. Not all faults will necessarily result in failures. For example, faults in the dead code will never result in failures. A fault that did not reveal failures may result in a failure when the environment is changed. Examples of these changes in environment include the software being run on a new computer hardware platform, alterations in source data, or interacting with different software. A single fault may result in a wide range of failure symptoms. Not all software faults are caused by coding errors. One common source of expensive defects is requirement gaps, that is, unrecognized requirements that result in errors of omission by the program designer. Requirement gaps can often be non-functional requirements such as testability, scalability, maintainability, performance, and security. Input combinations and preconditions A fundamental problem with software testing is that testing under all combinations of inputs and preconditions (initial state) is not feasible, even with a simple product. This means that the number of faults in a software product can be very large and defects that occur infrequently are difficult to find in testing and debugging. More significantly, non-functional dimensions of quality (how it is supposed to be versus what it is supposed to do) — usability, scalability, performance, compatibility, and reliability — can be highly subjective; something that constitutes sufficient value to one person may be intolerable to another. Software developers can't test everything, but they can use combinatorial test design to identify the minimum number of tests needed to get the coverage they want. Combinatorial test design enables users to get greater test coverage with fewer tests. Whether they are looking for speed or test depth, they can use combinatorial test design methods to build structured variation into their test cases. Economics A study conducted by NIST