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Chapter 19A Simulation

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 19A-3

Simulation-Defined A simulation is 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 traditional statistical and management science techniques 19A-4

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 Lets look at each of these steps in turn… 19A-5

Simulation Methodology:Problem Definition Specifying the objectives Identifying the relevant controllable and uncontrollable variables of the system to be studied 19A-6

Constructing a Simulation Model Specification of Variables and Parameters Specification of Decision Rules Specification of Probability Distributions Specification of Time- Incrementing Procedure 19A-7

Data Collection & Random No. Interval Example Suppose you timed 20 athletes running the 100-yard dash and tallied the information into the four time intervals below Seconds or more Tallies Frequency You then count the tallies and make a frequency distribution % Then convert the frequencies into percentages Finally, use the percentages to develop the random number intervals RN Intervals RN Intervals Accum. % You then can add the frequencies into a cumulative distribution 19A-8

Specify Values of Variables and Parameters Determination of starting conditions Determination of run length 19A-9

Run the Simulation By computer Manually 19A-10

Evaluate Results Conclusions depend on – 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 19A-11

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 19A-12

Proposing a New Experiment Consider changing many of the factors: – 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 19A-13

Considerations When Using Computer Models Computer language selection Flowcharting Coding Data generation Output reports Validation 19A-14

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 19A-15

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 19A-16

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 graphically the product flow through the system Permit interactive debugging 19A-17

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 19A-18

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 19A-19

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 19A-20

Question Bowl According to the Major Phases in a Simulation Study, what is the next step after you “run the simulation”? a.Define problem b.Evaluate results c.Validation d.Propose new experiment e.None of the above Answer: b. Evaluate the results 19A-21

Question Bowl Which of the following is a Key Factor in the “construct simulation model” step of the Major Phases in a Simulation Study? a.Specification of variables b.Specification of probability distributions c.Specification of time incrementing procedure d.All of the above e.None of the above Answer: d. All of the above (Correct answer can also include Specification of Decision Rules.) 19A-22

Question Bowl You have just tallied two intervals of time (i.e., 10 and 20 seconds) whose frequencies are 15 and 35, respectively. Which of the following are the Random Number Intervals if we were going to use these tallies for a simulation? a.1 to 15 & 16 to 35 b.00 to 14 & 15 to 34 c.00 to 29 & 30 to 99 d.01 to 15 & 16 to 99 e.None of the above Answer: c. 00 to 29 & 30 to 99 19A-23

Question Bowl You have just tallied three intervals of for events A, B, and C whose frequencies are 10, 40, and 50, respectively. Which of the following are the Random Number Intervals if we were going to use these tallies for a simulation? a.00 to 10 & 11 to 40 & 41 to 100 b.1 to 10 & 11 to 50 & 51 to 100 c.00 to 09 & 10 to 49 & 50 to 99 d.00 to 10 & 11 to 40 & 41 to 99 e.None of the above Answer: c. 00 to 09 & 10 to 49 & 50 to 99 19A-24

Question Bowl Which of the following are desirable features of simulation software? a.Interactive capabilities b.User-friendly c.Allowing users to write their own routines d.Macro capabilities e.All of the above Answer: e. All of the above 19A-25

Question Bowl Which of the following are advantages of using simulation? a.Better understanding of the real system to be modeled b.Does disrupt ongoing activities c.Unable to answer “what-if” questions d.All of the above e.None of the above Answer: a. Better understanding of the real system to be modeled 19A-26

Question Bowl Which of the following are disadvantages of using simulation? a.No guarantee to model will provide good answers b.Very costly c.Very time consuming d.No standard approach to simulation modeling e.All of the above Answer: e. All of the above 19A-27

End of Chapter 19A 19A-28