Pressures have been increasing for achieving a more finely tuned balance among product and process characteristics, including reliability. Trade-offs among product components with respect to reliability are also becoming increasingly important. This has made "seat-of-the-pants" engineering increasingly unacceptable. Quantitative approaches are essential. Thus an important use of software reliability measurement is in system engineering. However, there are at least four other ways in which software reliability measures can be of great value to the software engineer, manager, or user. First, you can use software reliability measure to evaluate software engineering technology quantitatively. New techniques are continually being proposed for improving the process of developing software, but unfortunately they have been exposed to little quantitative evaluation. Software engineers and managers greeted many of these innovations with enthusiasm at first, because of the strong need for them. Second, a software reliability measure offers you the possibility of evaluating development status during the test phases of a project. Methods such as intuition of designers or test team, percent of tests completed, and successful execution of critical functional tests have been used to evaluate testing process. None of these have been really satisfactory and some have been quite unsatisfactory. Third, you can use a software reliability measure to monitor the operational performance of software and to control new features added and design changes made to the software. The reliability of software usually decreases as a result of such changes. A reliability objective can be used to determine when, and perhaps how large, a change will be allowed. The following case study shows some of the users of software reliability measurement.

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