RASMUSSEN’S SRK MODEL BASIC DEFINITION AND TUTORIALS


The main thrust of this model is in the category of operator behaviour. It is based on Rasmussen’s theoretical work on the analysis of operator tasks. According to his model, three levels of operator behaviour may be identified.

Skill-based behaviour
This refers to routine tasks requiring little or no conscious attention during task execution. In this way enough ‘mental capacity’ is left to perform other tasks in parallel. For example: an experienced car driver travelling a familiar route will control the vehicle on a skill-based level, enabling them to have an intelligent discussion with a passenger, parallel to the driving task.

Rule-based behaviour
This refers to familiar procedures applied to frequent decision-making situations. A car driver integrating the known rules for right-of-way at crossings with stop signs or traffic lights, deciding whether to stop the vehicle or pass the crossing, is functioning at this level also.

The separate actions themselves (looking for other traffic, bringing the vehicle to a full stop, changing gears, etc.) will again be performed on a skill-based level. Making these familiar decisions and monitoring the execution of the skill-based actions requires some part of the total mental capacity available to the driver, but not all.

Knowledge-based behaviour
This refers to problem-solving activities, for instance when a person is confronted with new situations for which no readily available standard solutions exist. The same car driver approaching a crossing where the traffic lights have broken down during rush hour will first have to set their primary goal: do they want to proceed as fast as possible or do they want to minimize the chance of collision?

Depending on this goal they will control the vehicle with varying degrees of risk taking (e.g. by ignoring some of the usual traffic rules whenever they see an opportunity to move ahead somewhat). It is interesting to note that this model infers that an accident occurs as a consequence of goal setting followed by a human decision.

Any error may be attributed to human behaviour, perception, cognitive skills and experience, factors which have been widely used in other accident models.


Rasmussen has also developed the boundary theory which has particular relevance to large-scale acccidents. In this theory organizations operate in a space of possibilities within the three boundaries of economic failure, unacceptable workload and functionally acceptable performance.

Within this last boundary is the resulting perceived boundary of acceptable performance. The distance between these two is the margin for error. Rasmussen considers experiments to improve performance to create Brownian or random movements within this space.

There are pressure gradients operating, such as the gradient towards least effort, and the management pressure towards efficiency, both driving the organization in the direction of the perceived boundary of acceptable performance.

A counter gradient may exist in the form of safety culture campaigns. If corporate behaviour in the presence of strong gradients migrates past the perceived boundary and reaches the boundary of functionally acceptable performance, then an accident is likely.

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