The research work presented in this article concerns reengineering of the registered nurse diploma, particularly definition of Skills group No. 1: evaluate a clinical situation and establish a nursing diagnosis. It was designed to analyse real nursing practices in a typical clinical evaluation and diagnosis setting on admission of a patient to hospital. This analysis essentially focused on the cognitive processes used to organise nursing practices. The theoretical framework of professional training allowed this analysis to be performed on the basis of concepts of skills, schema and operative model. The research protocol focused on self-assessment interviews allowing explanation of the schemas used by a skilled nurse and a trainee nurse on admission of a patient in the day hospital for colonoscopy. An analysis of these schemas and especially the operative model of the skilled nurse demonstrated the organising concepts of effective nursing practice. The results, apart from their heuristic value to provide a better understanding of nursing professional practices, also provide resources to design training in nursing clinical evaluation.