Data plays an important part in the construction and use of a simulation model, and table 6.1 summarises the main types of data required (Pidd, 2009) at different stages of the simulation study. To understand the problem situation, we use what is termed contextual data. Along with assumptions we make about the real world, this provides us with an understanding of the real system known as the system description. We then undertake an abstraction process which makes simplifications of the real system to reduce model complexity. This includes defining the scope and level of detail of the model content, along with an explanatory model (process map) and a descriptive model (input modelling). The outcome of the conceptual modelling is termed realisation data, which we use to build our computer model. We will also use data for the verification and validation process covered in chapter 8.