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IB2110

SECTION A
Question 1
Complete the following statements (- each may require one or more words to complete):
a) In DES the system is represented by a series of …………….. .
b) VIS stands for ……………………. .
c) Discrete Event Simulations are good for modelling systems that are …………..………….. .
[Write your answers in the box provided below] (3 marks)

a) Events
b) Visual Interactive Simulation
c) complex, change over time (dynamic), have variability (randomness) in them, are
queuing systems.

Question 2 (18 marks in total)
Please read the following ‘story’ and then answer Questions 2a), b), c), d) and e) which are
based on this text:
SimULink Consultancy is a consultancy firm that specialise in conducting DES projects using
the Simul8 software. They have recently been hired by Portland Police Department (PPD)
Crime Laboratory.

Faced with tighter funding and increased demand, many forensic laboratories are examining
their processes to do more with their resources. DNA Units in particular have experienced a
sharp rise in demand in recent years as a result of the growing use of DNA testing as an
investigative tool. Between 2001 and 2011, the DNA Unit of the PPD Crime Laboratory saw
a 225% increase in its caseload. Despite expanding the laboratory to improve capacity, and
running the lab 24 hours a days, 7 days a week, using a staff shift system, the volume of
cases submitted for DNA testing continues to lead to longer turnaround times. Turnaround
times are the time taken from when a lab request is received until the results are reported.
The DNA Unit has target turnaround times of seven days for rush cases and 45 days for all
other case types.

With a growing backlog of work, identifying potential improvements is a challenge with
limited flexibility to assess operational changes that could help to further increase capacity.
The PPD Crime Laboratory is challenged by the fact that it only has a subjective view of its
processes, as well as limited data to assess solutions to improve efficiency. They do not
know, for instance, whether the existing laboratory equipment is being utilized at full
capacity, or whether there are areas that are ‘resource starved’, i.e. equipment waiting for
an analyst to become available to complete the task. So it is completely unclear to them
whether it is a lack of equipment or staff or both that is causing the backlog and long
turnaround times.
To overcome this challenge, Discrete Event Simulation has been chosen as a solution to
provide a more objective, evidence-based approach. The PPD Crime Laboratory turned to
SimULink Consultancy to thoroughly evaluate two potential investment options for
improving efficiency: purchasing additional equipment and hiring more staff. The PPD
Crime Laboratory wants to use the simulation to identify whether additional equipment
and/or staff could help alleviate the case backlog and achieve turnaround times.
The SimULink consultants have been hired to develop a simulation model that captures the
entire flow of the DNA laboratory processes that could then be turned over to the PPD
Crime Laboratory Improvement Project Manager within one month of the project start date.
It is important that the PPD Crime Laboratory Improvement Project Manager, who has no
previous simulation or computing experience, can use the simulation model easily to make
changes to the existing laboratory processes and hence evaluate the impact of adding
additional equipment and staffing on overall efficiency and turnaround times.
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The lab currently has 10 pieces of equipment. The PPD Crime Lab are confident that they
could accommodate twice this number in the existing building, if this was found to be
necessary to achieve their targets. They are also prepared to increase the current staff
numbers of 7 analysts, 1 lab technician and 2 supervisors to as many as necessary to reach
their targets.
The DES model is to be created in conjunction with the personnel working in the DNA unit to
help ensure that all steps in the laboratory processes are captured within the simulation. To
further validate the simulation, one year of real casework data will be provided to be
entered and ran through the simulation to compare the results and verify its accuracy. It is
important to the PPD that the final model should mimic the running of the lab for three
years, so that it will help them understand the long-term impact and the return on
investment of any additional resources as well as showing how long it will take to eliminate
the existing backlog.
It is therefore a priority to SimULink to produce a computer simulation that can complete a
model run in less than 4 minutes, to allow for efficient running of a large number of possible
scenarios by the PPD Crime Laboratory Improvement Project Manager. The PPD are also
intending to use the model to create buy-in from stakeholders, such as staff and funding
sources, for the proposed changes. They are also interested in re-using the model in the
longer term as a management and planning tool.

a) State the proposed clock time of the PPD Crime Laboratory Simulation.
[Write your answer in the box provided below] (1 mark)
Real time being modelled = 3 years
Question 2 continued over page....\
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Question 2 continued…\

b) State the proposed model duration of the PPD Crime Laboratory Simulation.
[Write your answer in the box provided below] (1 mark)

How long the model takes to run = less than 4 minutes
c) Write down in detail the modelling objectives of the PPD Crime Lab project as
described (including the proposed experimental factors).
[Write your answer in the box provided below and carry on over the page as
needed] (5 marks)
Question 2c) answer box continued over page....\
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Question 2c) answer box continued…\
Achievement:
To understand the long-term impact
and the return on investment of any additional resources
as well as showing how long it would take to eliminate the existing backlog
current utilization of resources,
(or otherwise explain the PPD overall aim)

Performance: so that target turnaround times are achieved of seven days for rush cases
and 45 days for all other case types.

Changes: by increasing staffing from current levels of 7 analysts, 1 lab technician and 2
supervisors to as many as necessary to reach their targets
and by increasing equipment numbers from 10 pieces of equipment to 20 (Constraint).

(marks are given for the correct information in logical format)

Question 2 continued…\

d) Write down in detail what model outputs should be monitored from the PPD Crime
Lab simulation model.
[Write your answer in the box provided below] (5 marks)

To determine achievement of objective
• Turn around times of rush cases
• Turn around times of other types of cases
• % utilisation of resources.
• Time to elimination of back-log

To identify reasons for failure to meet objective
• Bar chart of turn around times for each case
• Min, max, mean, variance of turn around times
• Time series of mean turn around time
• Number of cases waiting at each equipment – bottle necks
• Utilisation of staff
• Utilisation of equipment.

Question 2 continued…\

e) Write in detail, with explanations and justifications, the General Project Objectives of
the described PPD Crime Lab simulation project.
[Write your answer in the box provided below] (6 marks)

• Time Scale for project – produce validated, working computer simulation within
one month.

• Nature of model use:
– Flexibility: Model shouldn’t need to be changed beyond varying the 2
stated experimental factors: equipment and staff numbers.
– Run speed: Number of possible scenarios quite large, so need model to run
reasonably fast, at least 4 minutes/run

– Visual display: 2D or 3D model adequate (should be justified). Clients will
see and use model to create buy-in from stakeholders, so should be
recognisable as lab.

– Ease of use: Clients need to interact directly with model so consider
creating user interface, as Project manager is not simulation or computer
savvy.
– Foreseen reuse of model or components as a planning/management tool

Question 3
Explain clearly the differences between making simplifications and assumptions in DES
conceptual modelling and hence explain clearly when and why each would be made.
Clearly describe one example of each (one simplification and one assumption) that was (or
could have been) made in modelling the system in your group assignment. Justify why each
is an assumption or a simplification. State the system you modelled in your group work as
part of your answer.
[Write your answers in the box provided below and continue over the page as necessary]
(6 marks)

Simplifications are modelling decisions made in order to reduce complexity.
Whereas,
Assumptions are modelling decisions made because of uncertainties or beliefs about the
real system.

Looking for a clear appropriate example of one simplification and one assumption clearly
related to the group assignment project with clear and correct justifications of which they
are and why.

Marks can not be given if examples are incorrectly labelled as simplifications when they
are justifed as assumption and visa versa.

Question 4 over page....\

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Question 4
Explain what the method MSER-5 is used for in DES and why you would use it.
Hence, clearly explain and justify in detail why you would or would not have used MSER-5 or
an equivalent method with your group assignment model. State the system you modelled
in your group work as part of your answer.
[Write your answer in the box provided below] (4 marks)

MSER-5 is a warm-up method
for finding the trunction point/end of transient phase/ warm-up period length)
to illiminate the effect of the initial transient on the resulting output
for steady-state output.

Looking for clear correct argument as to whether the group work system model output
would have an initial transient and be steady state and therefore whether a warm-up
period is appropriate, or otherwise clear full arguments as to why MSER-5 is not
appropriate.

Question 5 (14 marks in total)
A DES model was constructed to investigate the functioning of a canteen. It currently has
one cold food counter which is self-service, followed by a checkout counter for cold food
only, manned by one dedicated assistant. It also has one hot food counter with one server,
followed by a checkout counter for hot food only, manned by one dedicated assistant. Both
cold and hot foods can be eat-in or take-away. Once the customer has paid they continue
into the seating area where they can either sit and eat or exit the canteen with their ‘take-
away’ food.
The canteen has been receiving rising numbers of customer complaints regarding the time
spent queuing to get food and pay for food. The manager wants to achieve 98% of
customers wating less than 2 minutes in a queue to choose food (hot or cold) and 98% of
customers waiting less than 1 minute in a queue to pay for the food (hot or cold) once
chosen.
The manager is willing to increase the number of servers at the hot food counter to a
maximum of three. He is also willing to put in one more (identical) cold food self service
counter.
The manager is also willing to increase the number of cold food checkouts (each with
dedicated assistant) up to a total of three. Likewise, he is willing to increase the number of
hot food checkouts (each with dedicated assistant) up to a total of three.
Alternatively he is willing to consider doing away with the dedicated hot and cold checkouts
and have one to five general checkouts instead that can serve anyone, with just one queue
for the multiple checkouts.
A floor plan of the current system is shown in Figure 5.1

Figure 5.1: Canteen current floor plan – not drawn to scale.
Question 5 continued over page....\

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Question 5 continued…\

Answer the following questions:
a) Draw two separate Activity Flow Diagrams, one for the current canteen system and
one for the altered canteen under the proposed change of creating four general
checkouts but keeping the cold and hot food counters unchanged. Clearly state and
justify any assumptions or simplifications you make (if any) and title the diagrams to
clearly indicate which system is being depicted.
[Write your answers in the box provided below] (10 marks)

No marks were given if a logic flow diagram was drawn instead.
Valid differences are allowed if justifiable and explained clearly.