More Vensim and âStuffâ Fall 2010

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# More Vensim and âStuffâ Fall 2010 - PowerPoint PPT Presentation

More Vensim and “Stuff” Fall 2010. TODAY. Recitation Lecture Hands-on. Recitation. Table lookups use _____ ______ between data points, by default. Comment about Table Lookups.

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### More Vensim and “Stuff” Fall 2010

TODAY
• Recitation
• Lecture
• Hands-on
Recitation
• Table lookups use _____ ______ between data points, by default
• To find the ordinate value corresponding to a particular abscissa value that is given, let b = ordinate desired, a given abscissa. Then

b = bi + (a – ai)*(bi+1 – bi) / (ai+1 – ai)

Where a has been determined to lie between abscissas ai and ai+1

How do physical processes in the actual system create lagged behavior?

How much disaggregation is necessary to represent the delay accurately?

Simulation Time Step
• Should be between .5 and .25 of the shortest time constant (delay) in the model
• Look at all of the time constants
• Perception time
• Delivery delay time
• Construction time
• Find smallest
• Set simulation time step appropriately
Integration Method
• Euler for models with discrete events
• RK4 for models with oscillation
Equilibrium
• Transient
Dynamic Test Inputs
• Purpose
• Reveal inherent behavior
• Create extreme conditions
• Examples
• Pulse
• Step
• Ramp—one we didn’t look at
• Exponential growth
• Noise—randomness
Generating surprises
• Test for asymmetric responses to positive and negative disturbances
• Test small and large amplitude inputs
• Test policies at multiple points in system
• Test multiple patterns of behavior
Extreme Conditions
• Purpose
• Reveal weaknesses
• Generate insight
• Methods
• Remove contents of stock with PULSE function
• Cut off inflows or outflows
• Artificially force variables to 0 or to infinity
Reality Check
• Purpose
• Automate model quality checks
• Format
• Test input
• THE CONDITION: Staff = 0
• Consequence
• IMPLIES: Production = 0
Partial Model Testing
• Purpose
• Divide and Conquer
• Develop understanding of subsystems
• Test response of subsystems to driving data
• Methods
• Cut & paste structures into a new model
• Use data variables or test inputs to drive behavior
Feedback Elimination
• Purpose
• Identify feedback loops that are causing behavior
• Methods
• Sever flow connections
• Replace variables with constants or test inputs
• Insert 0*… in equations
• Flatten lookups
Parameter Sensitivity Analysis
• Purpose
• Link behavior to feedback loop structure
• Identify leverage points
• Search for equilibria
• Methods
• Vary parameters and initial conditions
• Stretch and shift lookup table shapes
Types of Sensitivity
• Insensitive
• Pendulum always comes to rest at bottom
• Numerical
• Numerical values change, but behavior “looks” the same
• Behavior mode
• Shift from s-shaped growth to oscillation
• Policy
• Policy conclusions change
Policy Evaluation
• Purpose
• Develop effective policies
• Identify conditions for effectiveness
• Identify weakness in formulation of existing policies
• Tools
• Sensitivity Analysis
• Optimization
• Gaming
Automation
• Sensitivity analysis
• Calibration
• Optimization
• Command files