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