This work formed one main strand in the Verifier method which he developed with Joanne Hyde. This is a method for critically re-assessing previous research into difficult problems. The initial stage uses a range of elicitation methods to gain an accurate picture of the assumptions and normal working practices used in the previous work. The next stage uses a knowledge of experts’ behaviour and a range of error taxonomies to identify the places where human error is most likely to have occurred. The final stage uses various formalisms to assess whether or not an error actually has occurred. There is no guarantee that this method will catch every error – it is not a method for proving the correctness of a piece of previous work – but it improves the chances of finding key errors.