The process flow diagram (PFD) of methodology is shown in Figure 8.2. The SD model is a good method to deal with the real complex problem. It is a perspective and a set of conceptual tools that enable us to understand the structure and dynamics of complex systems. Also, it deals with nonlinear behaviors of dynamics. To capture the real picture, first a mental model known as a causal and loop diagram (CLD) is drawn. It consists of variables connected by arrows denoting the causal influence among them (Forrester, 1997). There are two types of loop in a CLD: one is the positive feedback loop (self-reinforcing) and the other is the negative feedback loop (self-correcting loop). It is also known as a qualitative part of SD modeling. Stock and flow diagrams (SFD) represent the quantitative part of SD. A CLD emphasizes the feedback structure of a system. An SFD highlights the underlying physical structure of a system. Stock represents the state of the system and flow represents the rate of increase or decrease of the stock (Sterman 2000).

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