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# More Vensim and Stuff 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

• Recitation

• Lecture

• Hands-on

• 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?

• 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

• Euler for models with discrete events

• RK4 for models with oscillation

• Transient

• Purpose

• Reveal inherent behavior

• Create extreme conditions

• Examples

• Pulse

• Step

• Ramp—one we didn’t look at

• Exponential growth

• Noise—randomness

• 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

• 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

• Purpose

• Automate model quality checks

• Format

• Test input

• THE CONDITION: Staff = 0

• Consequence

• IMPLIES: Production = 0

• 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

• 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

• 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

• 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

• Purpose

• Develop effective policies

• Identify conditions for effectiveness

• Identify weakness in formulation of existing policies

• Tools

• Sensitivity Analysis

• Optimization

• Gaming

• Sensitivity analysis

• Calibration

• Optimization

• Command files