In chapter 4, we covered the use of diagramming techniques to represent the explanatory model that provides a structural representation of the system. In general, pure explanatory models are seldom found, and in a DES model, we usually depend on descriptive sub-models to describe individual relationships and variability in an aggregated form. For conceptual modelling, when employing a descriptive model, the emphasis is on the collection and analysis of quantitative data from observations of system behaviour. There are a number of descriptive modelling (also termed input modelling in simulation studies) methods, but we may be constrained in our choice by the availability and time required for data collection. Greasley and Owen (2016) show examples from the literature of where descriptive approaches to modelling people’s behaviour have been applied to model different aspects of model content such as resource availability, process durations and decisions.