Internal models should cover all material risks of the company and should be consistent across all risks. Internal models should reflect the nature, scale and complexity of the underlying businesses; they should be proportional in sophistication to the materiality of the risks they cover. Materiality levels should be determined by stakeholders based on the model’s purpose. Practical considerations for models include usability, reliability, timeliness, process effectiveness, systems and cost efficiency. There should be an acceptable tradeoff between accuracy and the various practical constraints.