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# System Dynamics Modeling Overview Dr. R. MacKay. What is a Model?

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System Dynamics Modeling Overview Dr. R. MacKay

What is a Model?

Conceptual Models http://serc.carleton.edu/introgeo/conceptmodels/index.html Red Sunset

Physical Analog Model Make a model analogous to a real system.

Statistical Model Data is gathered from a population and information about probable mean, variance, and correlations are calculated.

Visualization Models Especially useful for processing and understanding large data sets

Mathematical Models Two basic types: 1) Analytical 2) numericalnumerical

Why are models useful?

Creating a model forces one to better understand the real system. Models can help explain observations and help guide the development of future experiments. Models can be extremely useful in explaining how a system works to yourself and to others. Experiments can be performed to help understand the response of a system to changes without harming the real system. A model is the only method that one can use to estimate the future behavior of a system to past, present, and future processes that may influence a system. Models can be extremely helpful in policy development. More ideas

Static vs Dynamic Models http://www.youtube.com/watch?v=P0Fi1VcbpAI

Model construction Identify a problem (global warming) and create a sketch of the expected behavior of the important variables over time. Try to identify policies that may improve the performance of the system (reduce global warming). Identify key variables (sun, temperature, atmosphere, CO2, Water Vapor,…) Obtain data related to these key variables. Create a simple model to emulate the systems behavior at present. You may want to start with the system at equilibrium and then add Use the model to explore how the system responds to changes in key variables. Identify weaknesses in the model and refine as needed.

Several additional quotes relevant to using models and developing theories include: "All models are wrong but some are useful" George E.P. Box "Make your theory as simple as possible, but no simpler." A. Einstein "For every complex question there is a simple and wrong solution." A. Einstein. Ockhams RazorOckhams Razor: "A rule in science and philosophy stating that entities should not be multiplied needlessly. This rule is interpreted to mean that the simplest of two or more competing theories is preferable and that an explanation for unknown phenomena should first be attempted in terms of what is already known." The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.

Units are very important See Appendix A Basic Math review in Appendix B

Systems Thinking

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Systems thinking a short definition for the person on the street Systems thinking is the opposite of scientific thinking.

The Blind Men and the Elephant It was six men of Indostan To learning much inclined, Who went to see the Elephant (Though all of them were blind), That each by observation Might satisfy his mind Godfrey SaxeGodfrey Saxe (1816-1887). http://en.wikisource.org/wiki/The_Blindmen_and_the_Elephant

System: a definition The behavior of a system depends not only on the behavior of the components of a system but also on how these components interact.

Systems Thinking (system components) Stocks can be thought of as storage bins where something accumulates. (Energy (temperature), water level in a bucket, bank balance, child height) Flows provide inflow or outflow for a stock

Systems Thinking (key ideas) Equilibrium (a stock is at equilibrium when the inflow matches the outflow) –Stability –Instability Time delay (everything takes time) Feedback processes The whole system is greater than the sum of the individual parts of the system.

Systems Thinking Systems of information-feedback control are fundamental to all life and human endeavor, from the slow pace of biological evolution to the launching of the latest space satellite … Everything we do as individuals, as an industry, or as a society is done in the context of an information feedback system. -Jay W. Forrester

Systems thinking Archetypes fixes that fail

Drilling more wells led to a decrease in water table and a decrease in available water. What started out as a quick-win ended up as a low leverage intervention A high leverage intervention would have been to thoroughly understand the water needs and resources into the future.

Systems thinking Archetypes fixes that fail

Systems Thinking (common behavior) No change Linear growth (loss) time

Systems Thinking (common behavior) Quadradic growth Exponential growth time

Systems Thinking (common behavior) Exponential decay

Systems Thinking (common behavior) Goal seeking

Systems Thinking (common behavior) Overshoot and collapse

Systems Thinking (common behavior) Oscillatory behavior

Why are models useful?

Creating a model forces one to better understand the real system. Models can help explain observations and help guide the development of future experiments. Models can be extremely useful in explaining how a system works to yourself and to others. Experiments can be performed to help understand the response of a system to changes without harming the real system. A model is the only method that one can use to estimate the future behavior of a system to past, present, and future processes that may influence a system. Models can be extremely helpful in policy development. More ideas

Static vs Dynamic Models http://www.youtube.com/watch?v=P0Fi1VcbpAI

Model construction Identify a problem (global warming) and create a sketch of the expected behavior of the important variables over time. Try to identify policies that may improve the performance of the system (reduce global warming). Identify key variables (sun, temperature, atmosphere, CO2, Water Vapor,…) Obtain data related to these key variables. Create a simple model to emulate the systems behavior at present. You may want to start with the system at equilibrium and then add Use the model to explore how the system responds to changes in key variables. Identify weaknesses in the model and refine as needed.

Several additional quotes relevant to using models and developing theories include: "All models are wrong but some are useful" George E.P. Box "Make your theory as simple as possible, but no simpler." A. Einstein "For every complex question there is a simple and wrong solution." A. Einstein. Ockhams RazorOckhams Razor: "A rule in science and philosophy stating that entities should not be multiplied needlessly. This rule is interpreted to mean that the simplest of two or more competing theories is preferable and that an explanation for unknown phenomena should first be attempted in terms of what is already known." The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.

Units are very important See Appendix A Basic Math review in Appendix B

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