A series of field observations over two years revealed that a significant number of the strategies employed by anaesthesiologists were of a preparatory and preventive nature. Instead of relying on on-site inspirations and complicated mental processing, anaesthesiologists used prior deliberations to prepare both mentally and physically, and to prevent troublesome situations from occurring. A framework was proposed to summarise the results of the field observations. With the guidance of the framework, anaesthesiologists' activities and verbal protocols during eight surgical operations were recorded and were analysed. A number of detailed behavioural patterns associated with anticipatory and preparatory activities were identified. In addition, discussions during four case rounds attended by anaesthesiologists were recorded and analysed. The results of the field study led to the development of a conceptual model of planning and action control in the interaction with a complex work environment. In this model, resources for actions are represented by Rasmussen's ``decision ladder,'' and prior deliberation, or planning, as a means for filling in missing components and changing goal structures in the decision ladder.

The results of the field study and the model developed serve to enlarge the scope of inquiry into the interaction between human and complex systems beyond the event-response unit. They depict the role of anticipation and preparation in the successful interaction with complex work environments. They illustrate an important category of problem solving situations and skills that have not received adequate attention before. They provide answers to some of the key questions for designing effective aids and training programmes. Moreover, they add to our understanding of the adaptive nature of human behaviour manifested as adopting a variety of strategies to overcome obstacles to achieving successful performance.