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More Vensim and “Stuff” Fall 2010 PowerPoint PPT Presentation


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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

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More vensim and stuff fall 2010 l.jpg

More Vensim and “Stuff” Fall 2010


Today l.jpg

TODAY

  • Recitation

  • Lecture

  • Hands-on


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Recitation

  • Table lookups use _____ ______ between data points, by default


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Comment about Table Lookups

  • 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


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In what two types of places can delays occur?


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What two types of delays are there?


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Comment about delays

The modeler must ask…

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

How much disaggregation is necessary to represent the delay accurately?


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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

    • Adjustment time

    • Perception time

    • Delivery delay time

    • Construction time

    • Lifetime

  • Find smallest

  • Set simulation time step appropriately


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Integration Method

  • Euler for models with discrete events

  • RK4 for models with oscillation


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Equilibrium

  • Transient

  • Steady-state


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Dynamic Test Inputs

  • Purpose

    • Reveal inherent behavior

    • Create extreme conditions

  • Examples

    • Pulse

    • Step

    • Ramp—one we didn’t look at

    • Exponential growth

    • Noise—randomness


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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


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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


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Reality Check

  • Purpose

    • Automate model quality checks

  • Format

    • Test input

      • THE CONDITION: Staff = 0

    • Consequence

      • IMPLIES: Production = 0


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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


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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


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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


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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


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Policy Evaluation

  • Purpose

    • Develop effective policies

    • Identify conditions for effectiveness

    • Identify weakness in formulation of existing policies

  • Tools

    • Sensitivity Analysis

    • Optimization

    • Gaming


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Automation

  • Sensitivity analysis

  • Calibration

  • Optimization

  • Command files


  • Login