Lecture 2
BSC 417/517
Today’s class
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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
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”
N umber of People
Birth
D eath
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:
menu
• FILE
• Open – can only have one model open at a time within a window
menu
• EDIT
menu
• MODEL
• Model prefs  all sorts of goodies here to play with like "animation"
menu
• RUN
Used to run models
• Run:
Specs: Where you set time units, duration, numerical methods,
• Time
simulation speed
Specs: Used for “sensitivity analysis”
• Sensi
• HELP menu
of useful information – use it!
• Ae.g.wealth
• 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 writeups
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:
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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
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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