Conceptual modelling, determining what to include in the model and at what level of detail, affects model performance directly. However, it has so far received little attention in the literature mainly because conceptual modelling is viewed as more of an art than science. This paper describes a study on conceptual modelling in which data on the time spent on the different topics during real simulation projects by an expert and by novice modellers was collected and analysed. There were several differences between the data for the expert and the data for the novices, possibly indicating a difference in the modelling style.