CONCEPTUAL MODELLINGMarzo 26 de 2012

Introduction A simulation of a fast-food restaurant could take many

forms. At the simplest level the model might include only the queues and service desks.

could be expanded to include the tables and seating area, the kitchen, the supply of raw materials, the drive thru, the car park and so on.

There is also a need to consider the level of detail at which each component is to be modelled.

The purpose of this and the next chapter is to describe the requirements for conceptual modelling and to describe how a simulation modeller might go about designing the conceptual model.

Conceptual Modelling: Important but Little Understood

Conceptual modelling is almost certainly the most important aspect of the simulation modelling process.

The model design impacts all aspects of the study The data requirements. The speed with which the model can be developed. The validity of the model. The speed of experimentation. The confidence that is placed in the model results.

A well designed model significantly enhances the possibility that a simulation study will meet its objectives within the required time-scale.

What sets truly successful modellers apart is their effectiveness in conceptual modelling

Simulation studies suggest that 50% of the benefit is obtained just from the development of the conceptual model.

The modeller needs to develop a thorough understanding of the operations system in order to design an appropriate model.

Some might argue that the emergence of modern simulation software has reduced, or even removed, the need for conceptual modelling.

What modern simulation software does provide is an environment for more rapid model development.

Could be argued that the power and memory of modern hardware and the potential for distributed software has increased the need for conceptual modelling.

Salt (1993) and Chwif et al. (2000) bemoan the increasing complexity of simulation models and the problems associated with them.

People build more complex models because the hardware and software enables them to.

There is surprisingly little written on the subject. Law and McComas, 2001 - How to build valid and credible simulation models. Proceedings of the 2001 Winter Simulation Conference (Peters, B.A., Smith, J.S.,

Medeiros, D.J. And Rohrer, M.W., eds). Piscataway, NJ: IEEE. pp. 2229.

The main reason for this lack of attention is no doubt that conceptual modelling is more of an art than a science and therefore it is difficult to define methods and procedures.

This chapter introduces the basic concepts of conceptual modelling.

First, the meaning of conceptual modelling is more precisely defined.

The requirements of a conceptual model are discussed.

The chapter concludes by discussing the reporting and communication of the conceptual model.

What is a Conceptual Model?

Zeigler (1976) sheds some light on the definition of a conceptual model by distinguishing between four terms: The real system is that which the simulation

model is to represent. The experimental frame is the limited set

of circumstances under which the real system has been observed

The base model is capable of accounting for the complete behaviour of the real system

The lumped model the components of the system are lumped together and the interconnections are simplified.

Conceptual Model Definition

The conceptual model is a non-software specific description of

the simulation model that is to be developed, describing the

objectives, inputs, outputs, content, assumptions and

simplifications of the model.

Two Key Features in the Definition

1.Specifically identifies the independence of the conceptual model from the software in which the simulation is to be developed.

2.The definition outlines the key components of the conceptual model, which are as follows:

Objectives: the purpose of the model and modelling project.

Inputs: those elements of the model that can be altered to effect an improvement in, or better understanding of, the real world; otherwise known as the experimental factors.

Outputs: report the results from simulation runs. Content: The components that are represented in the

model and their interconnections. Assumptions: Made either when there are uncertainties

or beliefs about the real world being modelled. Simplifications: Incorporated in the model to enable

more rapid model development and use (Section 6.3).

Assumptions are ways of incorporating uncertainties and beliefs about the real world into the model.

Simplifications are ways of reducing the complexity of the model.

The content of the model should be described in terms of two dimensions: The scope of the model: the model

boundary or the breadth of the real system that is to be included in the model.

The level of detail: the detail to be included for each component in the models scope.

The purpose of the conceptual model is to set out the basis on which the computer based simulation (computer model) is to be developed.

For many modellers there is a temptation to start coding the computer model as soon as possible. Without due attention to the

development of the conceptual model, however, this can lead to a model that does not achieve what is required

The model may have to be completely rewritten, wasting significant amounts of time.

Requirements of the Conceptual Model

Willemain (1994) lists five qualities of an effective model: validity, usability, value to client, feasibility and aptness for clients problem.

Brooks and Tobias (1996) identify 11 performance criteria for a good model.

Four main requirements of a conceptual model: Validity Credibility Utility Feasibility.

ValidityA perception, on behalf of the modeller, that the conceptual model will lead to a

computer model that is sufficiently accurate for the purpose at hand.

Underlying this notion is the question of whether the model is right.

The subject of validity is discussed in more detail in Chapter 12.

CredibilityA perception, on behalf of the clients, that the conceptual model will lead to a computer model that is sufficiently

accurate for the purpose at hand.

Is taken from the perspective of the clients rather than the modeller.

UtilityA perception, on behalf of the modeller and the clients, that the conceptual model will lead to a

computer model that is useful as an aid to decision-making within the specified context.

moves away from simply asking if the model is sufficiently accurate, to whether it is useful.

FeasibilityA perception, on behalf of the modeller and the clients,

that the conceptual model can be developed into a computer model.

Various factors may make a model infeasible. It might not be possible to build the proposed model

within the required time-scale. Data requirements of the model may be too onerous, or

there is insufficient knowledge of the real system to develop the proposed model.

Keep the model simple

Communicating the Conceptual Model

Background to the problem situation (Section 6.2.1). Objectives of the simulation study (Section 6.2.2). Expected benefits (Section 1.3.2). The conceptual model: inputs, outputs, content (scope

and level of detail), assumptions and simplifications (Chapter 6).

Experimentation: scenarios to be considered (Chapter 10).

Data requirements: data required, when required, responsibility for collection (Section 7.2).

Time-scale and milestones (Section 4.3). Estimated cost (Section 4.6).

There are four main reasons why it should be expected that the specification will change during a simulation study: Omissions in the original specification. Changes in the real world. An increased understanding of simulation on

behalf of the clients. The identification of new problems through

the development and use of the simulation model.

Representing the conceptual model

Component list Process flow diagram Logic flow diagram Activity cycle diagram

Component list

Process flow diagram (process map)

Logic flow diagram

Activity cycle diagram

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