1. Cairo University
Faculty of Computers & Information
Department of Decision Support
Simulation Software Packages
Final Exam 2005/2006
Duration: 3 hours
Instructors: Dr. Mohamed Saleh
Question 1:
1. Reference modes are used to visualize systems behavior over time, thus, help in describing
the problem. You have studied 5 families of system behavior; Linear, Exponential, Goal
seeking, Oscillation, and S-shaped families.
a) Draw a simple time plot to illustrate the behavior of each family.
b) Draw a diagram of the structure that generates a goal seeking behavior. (Stock and
flow diagram)
2. Your client team is worried about employee motivation and is debating the best ways to
generate maximum effort from their people. One group argues that the greater the
performance shortfall (gap between required performance and actual performance), the
greater the motivation of employees will be. The secret of this motivation is to set aggressive
goals to elicit (bring out) maximum motivation and effort from employees. The other group
argues that a too big performance shortfall will cause frustration and disappointment as
people conclude there is no chance to accomplish the goal.
a) Identify the key variables in this situation
b) Draw a causal loop diagram
Question 2:
1. Briefly explain the following concepts. Illustrate your answer with figures whenever
possible:
a)
b)
c)
d)
e)
Mental models
Shared vision
Interactive learning environments
Model testing.
Reference mode
2. The SIR model represents the structure of spreading of an infectious disease. It assumes that
people who recovered from the disease acquire permanent immunity against that disease.
However, for some diseases, immunity is not permanent. It decays over time, leaving
formerly immune people susceptible to re-infection.
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2. Modify the model to incorporate loss of immunity. Assume immunity decays at a rate
determined by an average duration of immunity. Add any modeling constructs you feel
necessary (stocks, flows, constants, or auxiliary variables). Write down the equation or value
(assume reasonable value in case of constants or initial values) of each of the modeling
constructs you added.
Question 3:
1. The BASS diffusion model was developed for the diffusion of new product growths, taking
into account the effect of WOM and advertising on product diffusion in the market. The
BASS model assumes that the total size of the market (total population, N) is constant. In
general, the population of a community can significantly change over time through births,
deaths, and migrations.
Modify the BASS model to incorporate growth in the size of the total population. Assume that
the Population is changed by a constant net change rate that incorporates births, deaths, and
migrations. Add any modeling constructs you feel necessary (stocks, flows, constants, or
auxiliary variables). Write down the equation or value (assume reasonable value in case of
constants or initial values) of each of the modeling constructs you added.
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3. 2. The year 1991 marked a series of firsts for PC-PLUS, an established IBM-PC clone
manufacturer. That year PC-PLUS experienced its first quarterly loss ($20 million), its first
round of layoffs (1700 people), and the first departure of chief executive. These were the not
the kind of records PC-PLUS was used to posting. In the late 1980s, the company created the
sensation in the business world by growing to $1 billion in sales faster than any other
American firm in history. But then the early 1990s saw PC-PLUS losing market share to
other clone makers and struggling to stay on top. PC-PLUS initial strategy had been simple:
Build IBM-compatible computers that cost about the same as the competition but that either
performed better or offered extra features. PC-PLUS's engineering strength, combined with
its marketing savvy (know how), jump started its early success. The company could
command premium prices by offering technologically sophisticated products. But in 1991,
customers began perceiving PC-PLUS's products as over priced, and questioned the
company's leadership role in engineering break through. In 1986, PC-PLUS had leaped ahead
of its biggest rival by bringing out the first IBM-compatible machine using new, faster
microchip. But in 1991, PC-PLUS sat on the sidelines while three other clone makers
announced their own new machines with an improved processor. These competitors asked
customers why they should pay PC-PLUS's high prices; and the customers listened.
To maintain its success and to reduce its financial pressure, PC-PLUS needed to approximate
its competitors' prices without losing its reputation for quality. PC-PLUS remedy plan was to
keep costs down by buying more parts from outsiders. A reduction in costs, would increase
PC-PLUS's profits, hence alleviating PC-PLUS's financial pressure. However, some analysts
worried that this plan would hurt the company's premium-brand image. PC-PLUS would
appear to be "just another clone", which would dramatically affect PC-PLUS's sales, which
in turn would significantly impact PC-PLUS's profits.
Draw a causal loop diagram for this situation using ONLY the following key variables:
financial pressure, use of outside supplier parts, profits, costs, brand image, and sales.
Question 4:
1. Discuss the following:
a) The two systems dynamics key functions
b) The dynamic hypothesis of the modeling process
2. Consider a stock with a single inflow rate R1 and a single outflow rate R2 as given in the
figure below. Assume the initial value of stock is 100 units. Draw the behavior of the stock.
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