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witness代写:系统建模-Why and How?

浏览: 日期:2019-03-17

  witness代写

  第1章:系统建模-Why and How?

  也许你认为科学是关于“解释,揭示自然的基本机制”(Waldrop,1992,第39页),但是建模比科学抽象要深入得多。相反,这是一项对任何通过适应来学习生存的生物体至关重要的技能。我们人类建立的大多数模型都在我们的头脑中(即心理模型),它们和本书主题的模型一样多。它们只是没有特别明确,除非我们恰好清楚地表达它们,并且构建模拟模型只是这种表达的一个例子。模型是在他们的建设者的头脑中构想出来的,在纸上和计算机上诞生并明确表示出来,当我们为了更好的东西而丢弃它们时,模型就会失效。

  这样的建模是有用的,因为我们的大脑可以同时处理的东西有一个非常小和有限的限制。通过形式化我们的心理模型并建立它们的显式模型,我们可以考虑我们正在研究的系统中更多的各种实体,以及它们之间更多的交互作用,即我们在范围和细节上都获得了。此外,为了生存,我们可以用这些模型做更多的事情,而不仅仅是预测;我们也可以开始描述和(最困难的)解释。事实上,描述、预测和解释有三个(相关的)目的,可以使用模型(Flood和Carson,1993)。描述是最直接的;基于观察,我们简单地构建了反映现实的模型。预测涉及更多,即使用此类模型预测未来行为,接受(通常是含蓄的,通常是危险的)过去的行为是一个良好指南的直觉(Casti,1994年)。然而,解释超越了两者,这确实是科学的本质。当然,给定的模型是否需要“解释性的……而不仅仅是预测性的”(Rivet,1972,pp 2-4),取决于它所支持的工作的目的。

  更重要的是,也许不是目的,但是,显式模型可以方便地分为两大类,即物理模型和抽象模型。前者可进一步划分为标志性模型或比例模型(如风洞中的模型飞机)和模拟模型(具有不同特性的模型);后者可分为分析模型(包括公式和方程相关变量)和数值模型(基于规则)。最后,模型可以是静态的或动态的(取决于它们是否随时间变化),然后是随机的(具有随机性或概率)或确定性的(没有)。这导致了动态建模的两个广泛领域:离散仿真和系统动力学。这两类构成了我们的主体。

  本介绍性章节应使您能够:

  了解建模系统背后的动机-理念及其背景

  使用丰富的图片、认知地图和影响图处理复杂性

  开始思考观点、范围和详细程度

  在特定情况下,在离散仿真和系统动力学之间进行选择。

  1.1 THE SYSTEMS APPROACH AND HOLISM

  At the core of building good models in operations management and the management sciences is the systems approach. This has a critical role because of three fundamental features.

  1. It recognises systems as wholes - unlike the reductionist approach, which presumes that understanding is best obtained by breaking the wholes down into their constituent parts. Ackoff (1993) commented that “it can be proven rigorously that when each part of an organisation improves its performance independently of other parts with which it interacts, the performance of the whole may well suffer. This follows from the fact that the performance of a system is not the sum of the performances of the parts taken separately, but is the product of their interactions”.

  2. It considers organisations as open systems in constant interaction with their environment.

  3. It is naturally comprehensible because human beings organise their knowledge into cognitive systems spontaneously (Jackson, 1991).In any model building, though, it is necessarily impossible to include all the elements of a given situation: there are too many and the interactions between them are too complex. The trick is thus to recognise the elements and their interactions not completely but adequately: this is often far from straightforward to achieve. Particularly in the early stages of modelling (Clarke and Tobias, 1995a; 1995b) we need ways of dealing with complexity (Flood and Carson, 1993) and to recognize why we are doing it. As illustrated in Figure 1.1 we will be hoping that, through informed discourse and debate, our modelling - and more generally, problem structuring and analysis – will result in rational decision making and interventions, and hence outcomes that are “desired” and somehow make the world a “better” place.

  1.2 DESCRIPTION, METAPHOR AND ANALOGY

  The approach in much management research is just to describe. Situation descriptions are framed in long and complicated textual terms, and are intended to provide a complete coverage of every possible feature of the situations without quantifying many (if any) of the properties involved. Indeed some organisational models “do little more than describe or depict” (Burke and Litwin, 1992). The trouble is that assessments of particular real situations against such models tend to be at best debatable and at worst arbitrary.

  More succinct than descriptions are metaphors, and these differ from analogies and from similes. Analogies reflect “correspondences between things in certain respects that are otherwise different” (Chambers Etymological English Dictionary) - e.g. “Money is like muck, not good unless it be spread” (Fowler’s Modern English Usage). Unlike similes (which are comparisons proclaimed plainly as such), metaphors are, in conversation at least, mere figures of speech based on comparisons that are tacit - e.g. “He is a tiger when roused” (Chambers).

  In management though, metaphors tend to assume the role of models and the use of metaphorical analysis in discussing organisational change has long been widespread (Grant and Oswick,1996; Palmer and Dunford, 1996). The "organisation as machine" metaphor can actually be traced back to Taylor (1911) but more recent studies tend to rely upon studies of biological systems for insights into the management of organisations (e.g. Waldrop, 1992). Indeed it has become popular, when attempting to understand almost anything in management, to refer to the animal kingdom (e.g. Collinvaux, 1978).

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