SYSTEM DYNAMICS  Dr. John Bradford
‘System-as-Cause’ ThinkingWhat causes the Slinky                         1. Laundry-list approachto oscillate?            ...
‘System-as-Cause’ Thinking• What causes the Slinky to oscillate?• THE SLINKY!   – The system itself is always the cause of...
Feedback• Feedback Loop: occurs when changes  generate effects that then influence the  original causes of the change, mak...
FeedbackTwo types of Feedback:                                        Positive Feedback1. Positive (reinforcing, amplifyin...
Causality and Feedback• Feedback and Self-reinforcing patterns  – Social phenomena can appear to take on a life of    thei...
Causality and Feedback• Feedback and Self-reinforcing patterns  – Example: English Colonialism spreads English    througho...
Causality and Feedback• When we think of causal relations, we  normally think of these 3 possibilities:  1. A may cause B ...
Causality and Feedback• There are also 3 additional explanations,  which we usually forget:  4. A and B may cause each oth...
Causality and FeedbackIrreversible Processes• Social research usually assumes that processes are   reversible: “if a chang...
Causality and FeedbackIrreversible Processes• Analogy:  –if someone is shot by a gun, removing   the bullet won’t make the...
‘System-as-Cause’ ThinkingFour assumptions that are usually wrong:  1. *Causes operate independently of each  other: (“lau...
Stocks and Flows                                             Stock                            f lowingStocks     “Nouns” ...
Stock-Flow diagram of Feedback   Stock-flow structure of “Positive” (Reinforcing) and    “negative” (counteracting) feedb...
Feedback loop Dynamics    When both positive and negative feedback are present in     the same system, four possibilities...
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  • A system is a set of interrelating, interconnected parts or elements that, together, generate some distinct outcome or behavior over time. In dynamical systems modeling, the behavior that the system exhibits over time is called itsdynamic.
  • Feedback occurs when the output of a system is also an input to that same system so that a change in a condition in one part of the system creates results elsewhere in the system that in turn change the original conditions.  Some examples of feedback concepts in the social sciences include vicious circles, self-fulfilling prophecies, homeostatic processes, and invisible hands  (Richardson 1991). Feedback implies circular, or reciprocal causal relations, where A influences B, and B in turn influences A, and so on. Note: the term “cybernetic” refers to goal-oriented or purpose driven systems. This includes all living organisms, and many non-living systems such as thermostats and heat-seeking missiles. These systems use negative feedback in the sense that they vary their output (behavior) so that the difference between their sensory inputs (perceptions) and their goals (‘reference standards’) is minimized.
  • This is another way of talking about positive feedback, aka reinforcing causal loops.
  • This is another way of talking about positive feedback, aka reinforcing causal loops.
  • See Ben GoertzelFrom Complexity to Creativity (1997: 200).
  • In the example used above, even if increases in relative poverty generated increases in crime, it’s not at all clear whether decreases in relative poverty would generate decreases in crime. Policies that make this assumption might fail, if they mistake an asymmetrical cause for a symmetrical one.
  • Stockscan physical or non-physical: non-physical stocks “states of being” like anger, self-esteem, trust, etc. Importantly, non-physical stocks need not obey the Law of Conservation- they are not zero-sum. Flows are not “inputs” to stocks; they do not “influence” them, and do not “have impacts” on them. Flows can by physical or non-physical. Non-physical flows may include processes like: learning, getting angry, communicating, etc.
  • 1 29-13 system dynamics

    1. 1. SYSTEM DYNAMICS Dr. John Bradford
    2. 2. ‘System-as-Cause’ ThinkingWhat causes the Slinky 1. Laundry-list approachto oscillate? – Gravity, – Removal of Hand 2. System-as-cause approach: – …. ?
    3. 3. ‘System-as-Cause’ Thinking• What causes the Slinky to oscillate?• THE SLINKY! – The system itself is always the cause of its own behavior. – “Mental models should contain only those elements whose interaction is capable of self-generating the phenomenon of interest" (Richmond 2010: 6).
    4. 4. Feedback• Feedback Loop: occurs when changes generate effects that then influence the original causes of the change, making subsequent change either more (+) or less (-) likely. Effect Cause
    5. 5. FeedbackTwo types of Feedback: Positive Feedback1. Positive (reinforcing, amplifying): Initial changes become amplified or magnified over time; patterns are Population Births reinforced. + + – Examples: exponential population growth; nuclear explosion; ‘rich getting richer’, etc.2. Negative (counteracting, Negative Feedback balancing): Initial changes are counteracted or Force of balanced out, so that conditions remain Jump up Gravity relatively stable. + - – Examples: homeostasis; a thermostat; “what goes up, must come down”, etc.
    6. 6. Causality and Feedback• Feedback and Self-reinforcing patterns – Social phenomena can appear to take on a life of their own. How does this happen? – Initial changes generate (other) conditions which encourage the original effect.
    7. 7. Causality and Feedback• Feedback and Self-reinforcing patterns – Example: English Colonialism spreads English throughout the world. England ceases to be a colonial power, but English is still a popular world language because English is spoken in the United States, which became a world power.
    8. 8. Causality and Feedback• When we think of causal relations, we normally think of these 3 possibilities: 1. A may cause B 2. B may cause A 3. A and B may both be caused by some other factor C
    9. 9. Causality and Feedback• There are also 3 additional explanations, which we usually forget: 4. A and B may cause each other (with no initial prompting other than random fluctuations) 5. A and B may cause each other, once initially activated by some other factor C 6. A and B may cause each other, in a process of mutual feedback with some other factor (or set of factors) C
    10. 10. Causality and FeedbackIrreversible Processes• Social research usually assumes that processes are reversible: “if a change in X makes Y change in a certain direction, then the opposite change in X would generate an opposite change in Y.”• Example: Poverty and Crime – Increases in poverty (X) tend to be associated with increases in crime (Y). – Assumption: “Therefore, a decrease in relative poverty will generate a decrease in crime.” – Even if the original cause was poverty, reducing poverty may or may not reduce crime!
    11. 11. Causality and FeedbackIrreversible Processes• Analogy: –if someone is shot by a gun, removing the bullet won’t make the wound go away.
    12. 12. ‘System-as-Cause’ ThinkingFour assumptions that are usually wrong: 1. *Causes operate independently of each other: (“laundry-list” thinking) 2. Causality runs one-way: (no feedback) 3. Effects are “linear” (fixed or proportional to their effect) 4. Effects are instantaneous (no lags or delays)
    13. 13. Stocks and Flows Stock f lowingStocks  “Nouns” that indicate conditions or states of being at a point in time.  Stocks are things that accumulate over time from flowsFlows  “Verbs” that represent activities or processes, which exist over time.  Flows fill and drain stocks, that is, they update the magnitude of stocks.
    14. 14. Stock-Flow diagram of Feedback Stock-flow structure of “Positive” (Reinforcing) and “negative” (counteracting) feedback systems: Reinforcing Loop: Counteracting Loop: Exponential growth Exponential decay Populat ion Populat ion declining growinggrowt h decline rat e ~ rat e A feedback loop occurs whenever a change in the magnitude of a stock in turn affects a flow into or out of that same stock.
    15. 15. Feedback loop Dynamics When both positive and negative feedback are present in the same system, four possibilities arise:1. exponential growth: the reinforcing loop will dominate the counteracting loop.2. exponential decay: the counteracting loop will dominate the dominate the reinforcing loop.3. equilibrium: they balance each other out.4. Oscillation: it goes up, then down, up and down, … Population growing declining growth rate decline ~ rate

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