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StockAndFlowBasics

StockAndFlowBasics - Fundamentals on notation behavior and...

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Fundamentals on notation, behavior, and simple feedback structures

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EMSE 235: Systems Thinking and Policy Modeling I 2 • Stock & flow notation • From Causal Loops to Stock & Flow Diagrams • Simple feedback structures & behavior • First exposure to iThink
Fundamentals on notation

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EMSE 235: Systems Thinking and Policy Modeling I 4 • Stocks represent accumulations, characterizing the state of the system. • Stocks also are the source of information for decisions and actions. • Stocks give systems inertia and provide them with memory .
Stock Inflow Outflow Converter Exogenous Converter EMSE 235: Systems Thinking and Policy Modeling I 5 Source (Sink) Valves (Flows) Information Link

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EMSE 235: Systems Thinking and Policy Modeling I 6 Stock Inflow Outflow Converter Exogenous Converter Inflow t ( ) = f 1 Converter t ( ) ( ) Outflow t ( ) = Constant or exogenous input Converter t ( ) = f 2 Stock t ( ) , ExogenousConverter t ( ) ( ) Stock t + dt ( ) = Stock t ( ) + Inflow t ( ) Outflow t ( ) ( ) dt Stock is an integrator (accumulator) of “Inflow-Outflow”. Other elements depend functionally on each other.
EMSE 235: Systems Thinking and Policy Modeling I 7 Stock Inflow Outflow Stock and Flow Diagram: Integral Equation: Differential Equation: Stock ( t ) = [ Inflow ( s ) Outflow ( s )] ds t 0 t + Stock ( t 0 ) Stock t = Net change in Stock = Inflow ( t ) Outflow ( t ) Inflow Outflow

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• Simulate means integrate the differential equations by discretizing time into steps of size DT (stepsize). • In simple models, Euler (integration) methods are used, models of higher order will sometimes require more advanced methods (Runge-Kutta). • All methods are provided by Vensim / iThink. EMSE 235: Systems Thinking and Policy Modeling I 8 Time DT 2DT 3DT 4DT n*DT 0
EMSE 235: Systems Thinking and Policy Modeling I 9 Inventory Production Shipments Pending Assignments New Assignments Finished Assignments Bank Account Deposits Withdraws Active Workers Hired Quit Exercise: Use VenSim/iThink to create these diagrams.

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EMSE 235: Systems Thinking and Policy Modeling I 10 Potential Customers Customers New Sales PotentialCustomers t + dt ( ) = PotentialCustomers t ( ) NewSales ( t ) dt Customers ( t + dt ) = Customers ( t ) + NewSales ( t ) dt One stock gains what the other stock lost.
EMSE 235: Systems Thinking and Policy Modeling I 11 Incorrect Correct Apples Apples Traded Orages Oranges Traded Apple to Orange Trading Rate Apples Magic Orages

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Modeling I 12 • The only way a stock can change is via its inflows and outflows. • In turn, the stocks may determine the flows. • Systems consist of network of stocks and flows linked by information feedbacks from the stocks to the rates. State of the System
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StockAndFlowBasics - Fundamentals on notation behavior and...

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