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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

Lecture 2

BSC 417/517

today s class
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
Course website:
  • http://bama.ua.edu/~brown/bsc417.html
software tools
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

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”

working in stella
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 stella9
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 stella10
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 stella11
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 stella12
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 stella13
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 stella14
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 stella15
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 stella17
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 stella18
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 stella19
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
Saving and showing your work
  • Screen shots
  • Structure and calculations in EXCEL
lview taking screen shots
LView: taking screen shots
  • Useful tool for homework/project write-ups
what s best
What’s Best!
  • Free version of LINDO
  • We’ll use this after the midterm
  • Download from linked website
using excel as modeling tool
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
Four components of a system
  • Reservoirs
  • Processes
  • Converters
  • Interrelationships
  • Stocks of things we’re interested in
  • Things that are accumulated, stored, passed on
  • Reservoirs can interact
processes flows
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
  • Rates of change
  • Differential
  • Dictates process impact on reservoirs
  • Complex or straightforward relationships between elements in the model
the difference equation
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 characteristics
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)


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

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