A first model (10), such as a architectural level model or an instruction set simulator model makes calls to a second model (12), such as a pipeline simulator for a data processing device returning cycle count or energy consumption values. The calls to the second model are relatively slow. The system stalls the returned behavioural characteristics from the second model (12) in a memo table (14) and when a sufficient number of these have been returned with sufficiently little variation between them, then they are marked as being valid for use in place of a call to the second model (12), thus speeding up modelling.