Seeing System Structure
In the previous section, we defined the concept of time paths and considered how these system behavioral patterns can be placed into one or a combination of five distinct categories. Now, we consider the source of these characteristic patterns. Specifically, we discuss the concept of cumulative (stock) and rate-of-change (flow) system variables. As the elemental building blocks of system feedback networks, "stocks" and "flows" virtually ensure that a dynamic system will behave in a difficult-to-understand or counterintuitive way. In addition, the role of nonlinear relationships in determining a system's limits, the strength or dominance of its feedback loops, and the complexity of its behavior,will be discussed.