例如，如果一个是建立一个串行系统的100个组成部分，其中每一个有一个可靠性。999的整体系统的可靠性将是0.999100 = 0.905。
The basic purpose of using software metrics is the effective management during the process control. Since Metrics strongly support software project management activities. They relate to the four functions of management as follows:
Many systems perform a task by having a single component perform a small part of it, and pass its result to another component in a serial fashion. The new component then performs a small piece of the task, and continues passing it along, until the task is completed. This is how people typically design programs, and hardware devices. The solutions can be cost effective and elegant. As Von Braun discovered, this makes it exceedingly difficult to build a reliable system. Such systems can have their reliability modeled using some equations. Thus building a serially reliable system is extraordinarily difficult and expensive.
For example, if one were to build a serial system with 100 components each of which had a reliability of .999 the overall system reliability would be 0.999100 = 0.905.
By utilizing redundancy, system component, hardware or software (provided the algorithms are diverse) can provide a boost in reliability. In the simplest case (fail silent components no correctness voting) only one of the redundant components must be working to maintain the system’s level of service.
For example, Consider a system built with 4 identical modules. The system will operate correctly provided at least one module is operational. If the reliability of each module is .95, then the overall system reliability is:
In this way, reliable system can be built despite the unreliability of its component parts, though the cost of such parallelism can be high. Models of more complex systems may be built by combining the simpler serial and parallel reliability models.