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Technical Note 16

Simulation

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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

OBJECTIVES

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Simulation

Defined

A simulationis a computer-based model used

to run experiments on a real system

Typically done on a computer

Determines reactions to different operating rules

or change in structure

Can be used in conjunction with traditionalstatistical 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

Run the simulation

Evaluate results

Validation

Propose new experiment

Stop From Exhibit TN16.1

Lets look at each

of these steps in

turn

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Simulation Methodology:Problem

Definition

Specifying 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 the100-yard dash and tallied the information

into the four time intervals below

Seconds0-5.99

6-6.99

7-7.99

8 or more

Tallies Frequency4

10

4

2

You then count the tallies and make a frequency

distribution

%20

50

20

10

Then convert the frequencies into percentages

Finally, use the percentages to develop the random number intervals

RN Intervals00-19

20-69

70-89

90-99

Accum. %20

70

90

100

You then can add the frequencies into a cumulative distribution

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Specify Values of Variables and

Parameters

Determination of starting conditions

Determination of run length

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Run the Simulation

By computer

Manually

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Evaluate Results

Conclusions depend on

the degree to which the model reflects the realsystem

design of the simulation (in a statistical sense)

The only true test of a simulation is how wellthe real system performs after the results ofthe 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 systemthey seek to model

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Considerations When Using Computer

Models Computer language selection

Flowcharting

Coding

Data generation

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

Be capable of being used interactively as well as allowing

complete runs

Be user-friendly and easy to understand

Allow modules to be built and then connected

Allow users to write and incorporate their own routines

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

Output standard statistics such as cycle times,

utilization, and wait times

Allow a variety of data analysis alternatives for both

input and output data

Have animation capabilities to display graphicallythe product flow through the system

Permit interactive debugging

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Advantages of Simulation

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

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

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 16