BSC 417/517

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Lecture 2

• BSC 417/517

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Todays 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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Course website

• http//bama.ua.edu/brown/bsc417.html

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Software tools

• STELLA (all-purpose modeling)
• Whats 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

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(No Transcript)
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Working in STELLA

• Note that our run-time version doesnt 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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Whats Best!

• Free version of LINDO
• Well 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 were 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

• Whats there what was there what came in
  what went out
• R(t1) R(t) SUM(in) SUM(out)
• R(tdt) R(t)(SUM(in) SUM(out))dt
• People on the island (tdt) 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