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The system utility may be defined in terms of the satisfaction of the system stakeholders. == Actual utility == The actual utility (or usefulness) is the actual stakeholders' satisfaction. 〔 () Zonnenshain & Harel, Task-oriented System Engineering, INCOSE Conference, Singapore, 2009 〕 The actual utility may be measured using customer satisfaction questionnaires. == The utility life cycle == The value of the actual utility is not static. It changes in time, according to the system maturity. The utility function has two phases: The startup and the main phase. * The startup phase begins with the initial usage of the system and ends with the utility function reaching maximum utility. The initial value of the utility function is determined by the intuitivity of the user interface. The slope from the initial value to the maximum value is determined by the learnability. * Following the startup phase is the beginning of the main phase, in which the utility function stabilizes. Then the system utility gradually decreases. The reasons for the utility decrease include hardware reliability and maintenance costs, well known in common QA. Additional reasons for the decrease in the system utility may be attributed to human factors, such as use errors and the user’s capability to handle system faults. The utility function may be described as in the following chart: 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「System utility (system engineering)」の詳細全文を読む スポンサード リンク
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