1. More Vensim and “Stuff” Fall 2010

2. 6/14/20162 TODAY Recitation Lecture Hands-on

3. 6/14/20163 Recitation Table lookups use _____ ______ between data points, by default

4. 6/14/20164 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 = b i + (a – a i )*(b i+1 – b i ) / (a i+1 – a i ) Where a has been determined to lie between abscissas a i and a i+1

5. 6/14/20165 In what two types of places can delays occur?

6. 6/14/20166 What two types of delays are there?

7. 6/14/20167 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?

8. 6/14/20168 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

9. 6/14/20169 Integration Method Euler for models with discrete events RK4 for models with oscillation

10. 6/14/201610 Equilibrium Transient Steady-state

11. 6/14/201611 Dynamic Test Inputs Purpose Reveal inherent behavior Create extreme conditions Examples Pulse Step Ramp—one we didn’t look at Exponential growth Noise—randomness

12. 6/14/201612 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

13. 6/14/201613 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

14. 6/14/201614 Reality Check Purpose Automate model quality checks Format Test input THE CONDITION: Staff = 0 Consequence IMPLIES: Production = 0

15. 6/14/201615 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

16. 6/14/201616 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

17. 6/14/201617 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

18. 6/14/201618 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

19. 6/14/201619 Policy Evaluation Purpose Develop effective policies Identify conditions for effectiveness Identify weakness in formulation of existing policies Tools Sensitivity Analysis Optimization Gaming

20. 6/14/201620 Automation Sensitivity analysis Calibration Optimization Command files

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