The essential vocabulary for systems thinking System: A collection of elements that are interrelated to achieve some purpose. Reference mode: A pattern of behavior that persists over time and is characteristic of the system. Time horizon: The interval of time over which the problem behavior is fully observed. Alternately, the length of time required for the consequences of today's policies to become fully manifest. Behavior over time graph: A diagram that shows the reference mode behaviors over the time horizon of one or more critical system elements. Causal link: The theory or observation that a change in one variable will tend to cause a change in another variable. Positive link: A causal link involved when a change in one variable causes the affected variable to change in the same direction (i.e. both will increase or both will decrease). For example, a rise in pork price will tend to cause an increase in the breeding herd of female pigs. Negative link: A causal link involved when a change in one variable causes the affected variable to change in the opposite direction (i.e. one variable will increase while the other decreases, or vice versa). For example, a rise in pork price will tend to cause a decrease in the per-capita consumption of pork.

Feedback loop: A closed chain of causal loops. A change in one variable will propagate around the loop to cause an additional change in that variable. Positive loop: A feedback loop in which change is reinforced, leading to exponential growth or decline.

Negative loop: A feedback loop in which change is counterbalanced, leading to exponential approach to the goal or to oscillation around the goal. Delay: A lag between cause and effect that is long enough to be visible in the context of the time horizon. Exponential growth: A process of change in which the variable changes by the same percent in a given time period. Asymptotic growth: A process of change in which the variable moves towards a goal or equilibrium value, typically approaching it by the same percentage during each time period. Leverage point: This is a factor or relationship in the system where a relatively small amount of change will produce major (hopefully desirable) changes in the system’s behavior. Dominant loop: This is the feedback loop which has the major influence on the system's behavior over a specified interval. The loop that is dominant during one period typically will not be dominant during another.

Shifting dominance: This is the phenomenon that occurs when the major influence over system behavior shifts from one loop to another. System boundary: A conceptual boundary within which are found all the key endogenous variables required to understand the behavior of interest. Exogenous variable: This is a variable that may influence the value of system elements, but it is not itself influenced by what happens in the system (at least over the time horizon of current interest). Endogenous variable: This is a variable that does influence the value of system elements, but it is itself influenced by what happens in the system. In other words, it is part of the causal loop structure of the system. Excluded variable: This is a variable that does not appear in the causal loop diagram. Of course the vast majority of all variables in real life must be excluded from our models. Model: This is a representation of the system. It may be physical in form, or graphical, or verbal, or mathematical. In any case it is a highly simplified portrait of reality that contains, hopefully, the necessary elements for understanding the problem of interest. No model is “true.” The important question is, “Is this model more useful for understanding a specific behavior than the other models that are available to me now?” Paradigm: This is a conceptual framework, or a filter, or a

theory that you use to interpret the information you obtain through your senses. It is inevitable that you will have a paradigm at each moment, but with mastery of systems thinking you will gain the ability to pick deliberately which paradigm you use. Game: A role-playing exercise in which there are roles, rules and a goal. Games may be competitive, cooperative, and recreational. We use games to illustrate how a set of rules, in combination with the psychology and knowledge of the participants, can generate a system’s behavior over time. Archetype: An archetype is a rather simple causal loop diagram that illustrates the main links and loops involved in producing a problem that appears in one form or another in many different locations. Escalation, addiction, eroding goals, and commodity production cycles are all examples of common archetypes. An archetype never explains 100% of the causes of a specific problem, but it provides an extremely useful starting point for understanding a system that exhibits the corresponding behavior.