文档介绍：软件需求分析的重要性
软件需求分析的困难性
软件需求分析的任务
软件需求分析的过程
软件需求分析的原则
结构化分析方法
原型化方法
补充:有关数据库的基本概念
第三章软件需求分析
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需求分析
需求分析是软件定义时期的最后一个阶段
回答“系统必须做什么?”的问题
系统分析员的主要焦点是“做什么(what)”,不是“怎样做(how)”
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需求分析的重要性
真的很重要吗?
例:Our real-time example is based on the embedded software in the Ariane-5, a space rocket belonging to the European Space Agency (ESA). On June 4, 1996, on its maiden flight, the Ariane-5 was launched and performed perfectly for approximately 40 seconds. Then, it began to veer off course. At the direction of an Ariane ground controller, the rocket was destroyed by remote control. The destruction of the uninsured rocket was a loss not only of the rocket itself, but also of the four satellites it contained; the total cost of the disaster was $500 million (Newsbytes home page 1996; Lions et al. 1996).
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The reason: there was no discussion in the requirements documents of the ways in which the Ariane-5 trajectory would be different from Ariane-4.
统计资料:
In 1994, the Standish Group surveyed over panies about their over 8000 software projects to find out how well they were faring. The results are sobering. Thirty-one percent of the software projects were canceled before they pleted. Moreover, in panies, only 9% of the projects were delivered on time and cost what they were budgeted, and 16% met those criteria in panies (Standish 1994).
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在美国高科技历史上曾有过令人痛心的事件:
大家知道,DEC曾经是美国三大计算机公司之一,几年前被康柏收购,从地球上消失,成为美国计算机界一大憾事。DEC曾以众多的高新技术著称于世。其中,它在最后的几年里研发出的 Alpha 计算机芯片更以卓越的技术在性能上超过了 Intel, sun 和其他厂家的芯片。微软也曾大力协助 DEC ,将 Windows Nt 移植到 Alpha 系统,然而,Alpha 在市场上彻底地失败了,成为 DEC 最终失败的原因之一。
为什么? 究其根本,还是因为 DEC 对其市场和用户不了解,盲目追求技术而铸成大错。Alpha 系统复杂,造价过高,绝大多数用户不能接受;而且它又不能与已有的系统兼容,使得在其上的软件开发相当困难,软件开发商也很难接受,所以 Alpha 把它的两级用户都得罪了,要想成功,谈何容易!
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需求错误是可以被检查出来的
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需求分析的重要性
在需求过程中会产生很多错误(事实3和4)。
许多错误并没有在早期被发现(事实2)。
这样的错误是能够在产生的初期被检查出来的(事实5)。
如果没有及时检查出来这些错误,软件费用会直线上升(事实1)
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