3 Designing Simulations that are Better than the Rest: Conceptual Modelling for Simulation Stewart Robinson Warwick Business School, University of Warwick, Coventry, CV4 7AL, United Kingdom Abstract In performing a simulation study the modeller needs to make decisions about what to include in the simulation model and what to exclude. The modeller is faced with the very difficult choice of determining what is the best model to develop. Make it too complex and it may not be possible to complete the model in the time available. Make it too simple and the results may not be sufficiently accurate. The process of determining what to model is known as conceptual modelling. In this paper we explore how a modeller can identify a best (or more accurately a better) model. Conceptual modelling is defined, a framework for conceptual modelling is outlined and the requirements of a conceptual model are discussed. Introduction One of the most difficult issues in simulation modelling is determining the content of the simulation that is going to be developed. The job of the modeller is to understand the real system that is the subject of the simulation study and to turn this into an appropriate simulation model. The chosen model could range from a very simple single server and queue, through to a model that tries to encapsulate every aspect of the system. In effect, there are an infinite number of models that could be selected within this range, each with a slightly different content. The question is: which model is best? We explore the answer to this question in this paper. On the surface we might suggest the answer is to build the model that contains as much detail as possible. After all, this model will be the closest to the real system and so surely the most accurate. This might be true if we had complete knowledge of the real system and a very large amount of time available to develop and run the model. But what if we only have limited knowledge of the real system and limited time? Indeed, we rarely have the luxury of vast quantities of either knowledge or time, not least because the real system rarely exists at the time of modelling (it is a proposed world) and a decision needs to be made according to a tight time schedule. So, if we need to develop a simpler model, we need to determine the level of abstraction at which to work. This process of abstracting a model from the real world is known as conceptual modelling. We shall define conceptual modelling and the process of doing it in more detail in a while, but first it is useful to have a practical illustration.