Sd final06 07_ds

Cairo University
Faculty of Computers & Information
Department of Decision Support

Simulation Software Packages
Final Exam 2006/2007

Duration: 3 hours

Solve only Six Questions (all questions have the same grade)

Question 1:
1. Discuss in-details the steps of the modeling process.

Question 2:
State whether the following statements are True or False. Justify your answer
a. Correlations among variables represent the structure of the system.
F, causation
b. As customer requirements change, the managers of the supply chain
respond by adjusting the rate at which resources are ordered and used
True.
c. An important limitation of causal loop diagrams is their inability to
capture the stock and flow structure of the system. true
d. For an epidemic to occur, each infective must, on average, pass the disease
on to more than one person prior to recovering. true
e. System Dynamics can be used to solve any kind of managerial problems.
True.

Question 3:
Identify the negative loops that might be responsible for the goal seeking behavior in the
following cases. What counter forces might prevent the state of the system from reaching
its goal?
- Television share of all advertising
- US traffic fatalities per vehicle mile
- Nuclear plant load factor

Question 4:
The following two quotes (el moqtbas) are actual interview transcripts (b el nas
“nsooos”) developed in fieldwork carried out in an automobile company. The managers,
from two different component plants in the same division of the company, describe why
the yield (the fraction of gross throughput that passes inspection & is usable) of their
lines was persistently (b esrar) low and why it had been so difficult to get process
improvement programs off the ground:
In the minds of the [operations team leaders] they had to hit their pack counts.
This meant if you were having a bad day and your yield had fallen… you had to run like
1

crazy to hit your target. You could say, “You are making 20% garbage, stop the line and
fix the problem,” and they would say, “I can’t hit my pack count without running like
crazy.” They could never get ahead of the game.
--Manager at Plant A
Supervisors never had time to make improvements or do preventive maintenance
on their lines… They had to spend all their time trying to keep the line going, but this
meant it was always in a state of flux (tadfoq) … Because everything was so
unpredictable. It was a kind of snowball effect that just kept getting worse.
--Supervisor at Plant B
.

Develop a single causal diagram capturing the dynamics described by the interviews.
Explain in a paragraph or two how the loops capture the dynamics described.

Question 5:
The figure below shows the trajectory of a stock. Determine the behavior of its net rate
by graphical differentiation.

S c (u its
to k n )

2 0
0

1 0
0

0
0

5

1
0

1
5

2
0

Question 6:
a) Draw the Stock and flow diagram of the Bass Diffusion Model.
b) Write all equations of the model
c) Even though the Bass model often works well, the model invokes a number of
restrictive assumptions. List as many restrictive assumptions as you can.

Question 7:
Consider the inventory model drawn below. Assume the production rate is equal to the
gap (between desired and actual inventory) divided by the adjustment time (AT). Given
the behavior of the desired inventory (shown below), sketch the behavior of the inventory

2

Production_Rate
Inventory
Desired_Inventory
Adjustment_Time

Desired_Inventory

and production rate. Assume that inventory initially equals the desired inventory & AT =
14.4 time units.

2,000

1,500

1,000
0

5

10 15 20 25 30 35 40

Time

Gap

 S(0) = s* - (s* - s(0))*e(-t/AT)
 th = 0.7*AT
Question 8:
Write all equations for the model portrayed in the following figure

I wish you a merry exam 
Dr. Mohamed Saleh

3

Sd final06 07_ds

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