Anaesthesia may be characterised as a paradigmatic field of activity in complex work environments. In our research, however, we shift the focus away from identifying performance deficiencies or classifying errors in how complexity is handled by the anaesthetists. Instead, our main focus is to investigate the operational structure of action sequences in a clinical setting.

Methods
Previous studies have faced the methodological problem of recording and interpreting simultaneous operations. This study, however, aims to counter this problem by applying a new observational method, sensitive to overlapping operations in action sequences. Due to the fact that (referring to the theoretical framework of activity theory) only operations are directly observable, our observation is located at the level of operations that constitute action sequences. Superimposed concepts, structuring a set of 41 observation codes (each of them representing a single operation), are monitoring, manual tasks, communication, documentation and additional operations outside the main remit of anaesthetists.

A computer-based recording technique, called 'FIT-System', provided us with a means to record overlapping operations online. To analyse the data, we conducted case studies focusing on the operational composition of action sequences. In order to quantify the relative amount of overlapping operations in a given time frame, measures of action density were calculated.

Results
The results of this study support the descriptive validity of action density as an indicator of the occurrence of overlapping operations. First, the results show that a reasonable amount of overlap is neglected by conventional observation methods. Second, action density due to overlapping operations shows a characteristic distribution, corresponding to different phases of anaesthesia. For example, periods characterised by increased action density include the induction of and the emergence from anaesthesia. Moreover, overlapping operations seem to have a central position when dealing with the various requirements related to the occurrence of unexpected events.

Discussion
Overlapping operations in action sequences may be interpreted as an indicator of coping with varying complexity during an anaesthesia case. These results fit very well with research on workload during the administration of anaesthesia. As this was an initial, exploratory investigation into the operational composition of action sequences, many areas of further study may be identified (e.g. differences between the action sequences of experts and novices, especially in cases of varying conditions). Moreover, we did not intend to analyse co-operational aspects in action sequences. This important aspect of clinical practice will be included in future research.

None of the material presented here should be interpreted as an argument for restricting the analysis of professional practice to the level of operations. Rather, we emphasise the need to integrate different levels of analysis. The integrative use of methods at various levels will provide a balanced system for investigating the anaesthetist's activity, taking into account specific occupational aspects. This will be crucial to assure patient safety in an increasingly complex health care system.

Paper presented at Measuring Behavior 2002 , 4^th International Conference on Methods and Techniques in Behavioral Research, 27-30 August 2002, Amsterdam, The Netherlands

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