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:

Software tools STELLA (all-purpose modeling) What’s BEST! (optimization modeling) – after the midterm – 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”

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

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