While designing interactive software, the use of a formal specification technique is of great help because it provides non-ambiguous, complete and concise notations. The advantages of using such a formalism is widened if it is provided by formal analysis techniques that allow to prove properties about the design, thus giving an early verification to the designer before the application is actually implemented. However, formal specification of interactive systems (even though aiming to produce reliable software) often does not address the issues of erroneous user behaviour. This paper tackles the problem by proposing a systematic way of dealing with erroneous user behaviour. We propose to extend task models (describing standard user behaviour) with erroneous user behaviour. Without appropriate support, incorporating erroneous user behaviour in task models requires much effort from task analysts. We thus propose the definition of patterns of user errors in task models. These patterns of errors are then systematically applied to a task model in order to build a task model covering both standard and erroneous user behaviour. These task models can then be exploited towards system models to provide a systematic way of assessing both system compliance to user tasks and system tolerance to user errors.

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Abstract. The quality of the design of an interactive safety-critical system can be enhanced by embedding data and knowledge from past experiences. Traditionally, this involves applying scenarios, usability analysis, or the use of metrics for risk analysis. In this paper, we present an approach that uses the information from incident investigations to inform the development of safety-cases that can, in turn, be used to inform a formal system model, represented using Petri nets and the ICO formalism. The foundations of the approach are first detailed and then exemplified using a fatal mining accident case study.