lecture 2 bsc 417/517. today’s class course website schedule & topics for rest of semester...
TRANSCRIPT
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Lecture 2
BSC 417/517
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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
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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
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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”
Number of People
Birth Death
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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
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Defining tools and terms in STELLA
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Defining tools and terms in STELLA
• Stocks (= State Variables)• Represent anything that accumulates or is
depleted over time• Describe the state of a system
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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
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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
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Defining tools and terms in STELLA
• Connectors• Indicate dependence• Indicate required inputs to equation for a
flow or converter (but not to a stock)
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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!
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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
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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)
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Defining tools and terms in STELLA
• Text boxes
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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
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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”
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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.
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Saving and showing your work
• Screen shots• Structure and calculations in EXCEL
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LView: taking screen shots
• Useful tool for homework/project write-ups
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What’s Best!
• Free version of LINDO• We’ll use this after the midterm• Download from linked website
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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
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Four components of a system
• Reservoirs
• Processes
• Converters
• Interrelationships
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Reservoirs
• Stocks of things we’re interested in
• Things that are accumulated, stored, passed on
• Reservoirs can interact
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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
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Converters
• Rates of change• Differential• Dictates process impact on reservoirs
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Interrelationships
• Complex or straightforward relationships between elements in the model
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A few useful concepts for the homework
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The island community system
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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
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Systems thinking: an introduction
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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
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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
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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