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Technical Note 17
Simulation
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OBJECTIVES
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Definition of Simulation
Simulation Methodology
Proposing a New Experiment
Considerations When Using Computer
Models
Types of Simulations
Desirable Features of Simulation
Software
Advantages & Disadvantages of
Simulation
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Simulation-Defined

A simulation is a computer-based
model used to run experiments on
a real system
–
–
–
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Typically done on a computer
Determines reactions to different
operating rules or change in
structure
Can be used in conjunction with
traditional statistical and
management science techniques
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Major Phases in a
Simulation Study
Start
Define Problem
Construct Simulation Model
Specify values of variables and parameters
Lets look at each
of these steps in
turn…
Run the simulation
Evaluate results
Validation
Propose new experiment
Stop
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From Exhibit TN17.1
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Simulation Methodology:Problem
Definition
 Specifying
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the objectives
 Identifying
the relevant
controllable and uncontrollable
variables of the system to be
studied
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Constructing a Simulation Model

Specification of Variables and
Parameters

Specification of Decision Rules

Specification of Probability
Distributions

Specification of Time-Incrementing
Procedure
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Data Collection & Random No. Interval
Example Suppose you timed 20 athletes running the
8
100-yard dash and tallied the information
into the four time intervals below
You then count the tallies and make a frequency
distribution
Then convert the frequencies into percentages
You then can add the frequencies into a cumulative distribution
Finally, use the percentages to develop the random number intervals
Seconds Tallies
0-5.99
6-6.99
7-7.99
8 or more
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Frequency
4
10
4
2
%
20
50
20
10
Accum. %
20
70
90
100
RN Intervals
00-19
20-69
70-89
90-99
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Specify Values of Variables
and Parameters
 Determination
of starting
conditions
 Determination
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of run length
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Run the Simulation
 By
computer
 Manually
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Evaluate Results

Conclusions depend on
–
–
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the degree to which the model reflects the
real system
design of the simulation (in a statistical
sense)
The only true test of a simulation is how
well the real system performs after the
results of the study have been
implemented
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Validation

Refers to testing the computer
program to ensure that the
simulation is correct

To insure that the model results are
representative of the real world
system they seek to model
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Proposing a New Experiment

Consider changing many of the
factors:
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
parameters
variables
decision rules
starting conditions
run length
If the initial rules led to poor
results or if these runs yielded new
insights into the problem, then a
new decision rule may be worth
trying
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Considerations When Using
Computer Models

Computer language selection
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Flowcharting
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Coding

Data generation
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Output reports

Validation
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Types of Simulation Models

Continuous
– Based on mathematical equations
– Used for simulating continuous
values for all points in time
– Example: The amount of time a
person spends in a queue

Discrete
– Used for simulating specific values
or specific points
– Example: Number of people in a
queue
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Desirable Features of Simulation
Software
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Be capable of being used interactively as well as
allowing complete runs
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Be user-friendly and easy to understand
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Allow modules to be built and then connected
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Allow users to write and incorporate their own
routines
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Have building blocks that contain built-in
commands

Have macro capability, such as the ability to
develop machining cells
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Desirable Features of Simulation
Software

Have material-flow capability
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Output standard statistics such as cycle
times, utilization, and wait times
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Allow a variety of data analysis alternatives
for both input and output data

Have animation capabilities to display
graphically the product flow through the
system

Permit interactive debugging
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Advantages of Simulation
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Often leads to a better understanding
of the real system
Years of experience in the real system
can be compressed into seconds or
minutes
Simulation does not disrupt ongoing
activities of the real system
Simulation is far more general than
mathematical models
Simulation can be used as a game for
training experience
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Advantages of Simulation
(Continued)

Simulation provides a more realistic
replication of a system than mathematical
analysis
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Simulation can be used to analyze transient
conditions, whereas mathematical
techniques usually cannot

Many standard packaged models, covering
a wide range of topics, are available
commercially

Simulation answers what-if questions
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Disadvantages of Simulation
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There is no guarantee that the model will, in fact,
provide good answers
There is no way to prove reliability
Building a simulation model can take a great deal
of time
Simulation may be less accurate than
mathematical analysis because it is randomly
based
A significant amount of computer time may be
needed to run complex models
The technique of simulation still lacks a
standardized approach
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End of Technical
Note 17
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