Quality can determine a software product's success or failure in today's competitive market. Among the many characteristics of quality, some aspects deal directly with the functional correctness or the conformance to specifications, while others deal with usability, portability, and so on. Even for market segments in which new features and usability take priority, such as software for personal use in the mass market, correctness is still a fundamental part of the users' expectations. Various evaluation models can help people determine whether a particular application conforms to recruitments and specifications. Quality-evaluation models can quantitatively evaluate selected quality characteristics or subcharacteristics to help one obtain a more objective assessment. Failure, fault, and error are all key to the correctness aspect of software quality. According to standard IEEE definitions, a failure occurs when program behavior deviates from user expectations; a fault is an underlying cause within software that could lead to certain failures; and an error is a missing or incorrect human action that could inject certain faults into the product. Failures, faults, and errors are often collectively called defects.