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Lecture 2

Lecture 2. BSC 417/517. Today’s class. Course website Schedule & topics for rest of semester Software tools and introductions Basic system components and characteristics Systems thinking Thinking in stocks and flows: practice. Course website:. http://bama.ua.edu/~brown/bsc417.html.

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Lecture 2

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  1. Lecture 2 BSC 417/517

  2. Today’s class • Course website • Schedule & topics for rest of semester • Software tools and introductions • Basic system components and characteristics • Systems thinking • Thinking in stocks and flows: practice

  3. Course website: • http://bama.ua.edu/~brown/bsc417.html

  4. Software tools • STELLA (all-purpose modeling) • What’s BEST! (optimization modeling) – after the midterm • http://www.lindo.com/products/wb/wbm.html • Microsoft Excel • For use in defining terms, units, and setting up systems of equations • Lview • For taking screen shots to use in homework/project write-ups

  5. What is STELLA? Systems Thinking Experiential Learning Laboratory with Animation! Icon based dynamic simulation modeling software How is it useful? Context for simultaneously building conceptual & mathematical model that responds to: Internal structure of the system External forces (“forcing functions”) acting on the system Tool for the practice/philosophy/worldview of “systems thinking”

  6. Working in STELLA • Note that our run-time version doesn’t allow for the saving of files • Your manipulations must be explained in your Excel file • Screen shots help

  7. Defining tools and terms in STELLA

  8. Defining tools and terms in STELLA • Stocks (= State Variables) • Represent anything that accumulates or is depleted over time • Describe the state of a system

  9. Defining tools and terms in STELLA • Flows (= differential equations) • Contain equation describing rate of growth or loss (flow/time) • Can be "uniflow" (one directional) or "biflow" (two directional) depending on what is logical • The "cloud" symbol indicates unspecified source or sink for flow; clouds indicate bounds of your model -- what's included, what's not

  10. Defining tools and terms in STELLA • Converters • a. Forcing functions (= external factors or conditions that influence internal dynamics) • b. Data for comparison ("calibration", "validation") • c. Numerical constants, coefficients, parameters • d. Equations (used for a, b or c) • e. Graphical relationships between variables (e.g. stimulus-response) • f. Various other odds and ends

  11. Defining tools and terms in STELLA • Connectors • Indicate dependence • Indicate required inputs to equation for a flow or converter (but not to a stock)

  12. Defining tools and terms in STELLA • Dynamite • Used to eliminate unwanted parts of model • Be careful! Don't release the mouse until you are sure you are blowing up the correct building block!

  13. Defining tools and terms in STELLA • Ghosts • Copies of stocks, flows, converters • Used to keep conceptual models interpretable and to avoid the "spaghetti phenomena" • Can not have connectors pointing in, only pointing out

  14. Defining tools and terms in STELLA • Graphs & Tables • Used to display the value of variables over time (time series) • Used to display relationships among variables (e.g. x-y scatterplots)

  15. Defining tools and terms in STELLA • Text boxes

  16. Defining tools and terms in STELLA • Views: • Map/Model toggle (Globe and X2 icons)  leave in "X2" mode for now • Interface, Map/Model, Equation toggle (arrows above Map/Model toggle)  leave in Map/Model for now

  17. Defining tools and terms in STELLA • Menus: • FILE menu • Open – can only have one model open at a time within a window • EDIT menu • MODEL menu • Model prefs  all sorts of goodies here to play with like "animation" • RUN menu • Run: Used to run models • Time Specs: Where you set time units, duration, numerical methods, simulation speed • Sensi Specs: Used for “sensitivity analysis” • HELP menu • A wealth of useful information – use it! • e.g. See section, “Controls on model construction layer”

  18. Defining tools and terms in STELLA • Simplified steps in model building with STELLA (this is a quick start, much more on this later) • Define the system of interest (formalize your conceptual model of how the system works) • Define key stocks and flows • Build a conceptual map of the model in STELLA (stocks, flows, converters, connectors) • Create equations, initialize stocks • Mentally simulate model – sketch out dynamics you anticipate on paper • Create graphs and run model • Seek to understand differences between mental and mathematical simulations • Ask “what if questions”, compare with real data, etc.

  19. Saving and showing your work • Screen shots • Structure and calculations in EXCEL

  20. LView: taking screen shots • Useful tool for homework/project write-ups

  21. What’s Best! • Free version of LINDO • We’ll use this after the midterm • Download from linked website

  22. Using EXCEL as modeling tool • I assume you all are proficient in EXCEL • Use EXCEL to: • Define terms • Define units • Explore relationships between variables • Keep track of work across sessions in STELLA • Clearly articulate the assignment

  23. Four components of a system • Reservoirs • Processes • Converters • Interrelationships

  24. Reservoirs • Stocks of things we’re interested in • Things that are accumulated, stored, passed on • Reservoirs can interact

  25. Processes (flows) • What adds to or subtracts from stocks • Rate is controlled by converters • Drinking is the process by which the keg goes dry • Could be fast or slow – depends on the number of people and how motivated they are

  26. Converters • Rates of change • Differential • Dictates process impact on reservoirs

  27. Interrelationships • Complex or straightforward relationships between elements in the model

  28. A few useful concepts for the homework

  29. The island community system

  30. The difference equation • What’s there = what was there + what came in – what went out • R(t+1) = R(t) + SUM(in) – SUM(out) • R(t+dt) = R(t)+(SUM(in) – SUM(out))dt • People on the island (t+dt) = people on the island before(t)+(birth flow – death flow)dt

  31. Systems thinking: an introduction

  32. Systems thinking: characteristics • Begins with the global, moves toward the specific • Focuses on dynamic processes • Seeks a closed-loop explanation for how things work • Identifies FEEDBACK LOOPS • Looks for checks and balances, potential for runaway processes • Focuses on causal relationships

  33. Stocks (reservoirs) and flows (processes) Outline: Model of water in a reservoir Conceptual model Physical model Dynamic simulation model/tour of STELLA Practice thinking in stocks and flows

  34. The language of dynamic simulation Stock = state variable: Amount of stuff stored in various parts of a system Flow = input & output = rate of change = differential equation: The movement of stuff into or out of a stock

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