Software Metrics and Reliability

The IEEE defines reliability as “The ability of a system or component to perform its required functions under stated conditions for a specified period of time.” To most project and software development managers, reliability is equated to correctness, that is, they look to testing and the number of “bugs” found and fixed. While finding and fixing bugs discovered in testing is necessary to assure reliability, a better way is to develop a robust, high quality product through all of the stages of the software lifecycle. That is, the reliability of the delivered code is related to the quality of all of the processes and products of software development; the requirements documentation, the code, test plans, and testing. Software reliability is not as well defined as hardware reliability, but the Software Assurance Technology Center (SATC) at NASA is striving to identify and apply metrics to software products that promote and assess reliability. This paper discusses how NASA projects, in conjunction with the SATC, are applying software metrics to improve the quality and reliability of software products. Reliability is a by-product of quality, and software quality can be measured. We will demonstrate how these quality metrics assist in the evaluation of software reliability. We conclude with a brief discussion of the metrics being applied by the SATC to evaluate the reliability . I. Definitions Software cannot be seen nor touched, but it is essential to the successful use of computers. It is necessary that the reliability of software should be measured and evaluated, as it is in hardware. IEEE 610.12-1990 defines reliability as “The ability of a system or component to perform its required functions under stated conditions for a specified period of time.” IEEE 982.1-1988 defines Software Reliability Management as “The process of optimizing the reliability of software through a program that emphasizes software error prevention, fault detection and removal, and the use of measurements to maximize reliability in light of project constraints such as resources, schedule and performance.” Using these definitions, software reliability is comprised of three activities: 1. Error prevention 2. Fault detection and removal 3. Measurements to maximize reliability, specifically measures that support the first two activities There has been extensive work in measuring reliability using mean time between failure and mean time to failure.[1] Successful modeling has been done to predict error rates and