Generic Structures First-Order Negative Feedback

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14D-4475-2DOCUMENT: This is the time constant. It represents the adjustment time for the stock.UNIT: timeFrom the comparison of the two possible equations for the rate, we notice that themultiplier in the rate equation is given by 2multiplier in the rate equation ? draining fraction ?1time constant2 For a more in depth explanation of time constants see the paper Beginner Modeling Exercises Section 3:Mental Simulations of <strong>Negative</strong> <strong>Feedback</strong> by Helen Zhu. (D-4536)
D-4475-2 153.3 Model BehaviorThe characteristic feature of exponential decay is its constant halving time which isthe time for the gap to be cut in half. Because the draining fraction is constant at 0.2, thehalving time remains constant for the entire simulation of the model. In Figure 6, forexample, the initial stock is 100 and the goal is 50, making the gap equal to 50. It takesabout 3.5 years for the gap to be cut in half to 25 (stock = 75) and another 3.5 years forthe gap to be cut in half again to 12.5 (stock = 62.5)!1: STOCK 2: GOAL FOR STOCK100.001150.0012 2 2120.000.00 5.00 10.00 15.00 20.00TimeFigure 6. Results of Simulation4. Behaviors produced by the generic structureThe behavior produced by this structure will vary, depending on the values of threeparameters: the initial value of the stock, the goal, and the draining fraction.4.1 Changing the initial value of the stockThis section explores the effect of varying the initial stock value. In our genericmodel, the stock has initial values of -8000, -4000, 0, 4000, and 8000 for runs 1 through 5respectively. The goal is constant and zero, the draining fraction at 0.2.
Page 1 and 2: D-4475-2 1Generic Structures:First-
Page 3 and 4: D-4475-2 3Table of Contents1. INTRO
Page 5 and 6: D-4475-2 5
Page 7 and 8: D-4475-2 71. IntroductionGeneric st
Page 9 and 10: D-4475-2 9A simple negative feedbac
Page 11 and 12: D-4475-2 11number of employeesfirin
Page 13: D-4475-2 13flow = adjustment gap *
Page 17 and 18: D-4475-2 171: STOCK 2: STOCK 3: STO
Page 19 and 20: D-4475-2 195. Summary of important
Page 21 and 22: D-4475-2 21To sum up, asymptotic gr
Page 23 and 24: D-4475-2 23Figure 12. Company Downs
Page 25 and 26: D-4475-2 25UNIT: atomsOUTFLOWS:rate
Page 27 and 28: D-4475-2 27Vensim Examples:Generic
Page 29 and 30: D-4475-2 292.2 Example 2: Populatio
Page 31 and 32: D-4475-2 31The initial time for the
Page 33 and 34: D-4475-2 33(05) GOAL FOR STOCK=a co
Page 35 and 36: D-4475-2 35Graph of Stock with diff

Generic Structures First-Order Negative Feedback

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