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First published online 11:21 GMT 11th March 2002, Copyright Derek J. Smith (Chartered Engineer). This version [HT.1 - transfer of copyright] dated 18:00 12th January 2010
Rasmussen's (1983) "Typical Levels of Performance" Omega
The Danish engineer Jens Rasmussen produced the following diagram in an attempt to apply better cognitive theory to the design of man-machine interface systems and thus help reduce the potential for accidents. He began by characterising human performance in a familiar environment as goal-oriented, and rule-controlled, and he then proposed three qualitatively different levels of cognition in which qualitatively different types of information circulate and qualitatively different types of decision are made. He referred to these as "typical levels of performance" (p258). This analysis was subsequently adopted without fundamental alteration in James Reason's (1990) book "Human Error". Here is Rasmussen's diagram and a summary of his proposals:
Rasmussen's (1983) Three Levels of Skilled Performance: This is Rasmussen's (1983) attempt to distinguish between the three levels of the standard five-box omega, (a) by the nature of the behaviour controlled by each hierarchical processing layer, and (b) by qualitative differences in the nature of the control information itself. Here are the three levels he proposes:
Rasmussen describes the simplest form of behaviour as skill-based behaviour (SBB). It is controlled from the lowest level of the cognitive processing hierarchy, and may be characterised as "smooth, automated, and highly integrated" (p258) and takes place (critically) "without conscious attention or control" (p259). Effective SBB performance relies on heavy feedforward control flows throughout, "depends upon a very flexible and efficient dynamic internal world model" (p259), and will usually involve rapid coordinated movements. Examples of SBBs are bicycle riding or musical performance. As for the nature of the information at this level, SBB is described as relying on signals, which are defined as "representing time-space variables from a dynamical spatial configuration in the environment" (p261).
ASIDE: If unfamiliar with the difference between "feedforward" and "feedback" information in a control system, click here.
The next level of complexity is rule-based behaviour (RBB). It is controlled by the middle level of the processing hierarchy, and may be characterised as consisting of "a sequence of subroutines in a familiar work situation" (p259), where the subroutines follow previously stored rules, again relying primarily on feedforward control. Examples of RBBs are mathematical problem solving and system control tasks such as the discrete manoeuvring of aircraft or cars. As for the nature of the information at this level, RBB is described as relying on signs to indicate the state of the environment. These are defined as "related to certain features in the environment and the connected conditions for action" (p261).
ASIDE: Rasmussen explicitly warns that "the boundaries between skill-based and rule-based performance is not quite distinct" (p259), varying with both level of training and attentional state. The acid test is this: rule-based control is ultimately based upon "explicit know how" - the rules can be explained in words by the person concerned; so if you cannot explain it then it must be skill-based.
The highest level of complexity is knowledge-based behaviour (KBB). It is controlled by the highest level of the processing hierarchy, relies upon a "mental model" of the system in question, and in general terms is to be strongly avoided because what it achieves in terms of sophistication it loses in the time it takes [for an example of what happens when all training fails and things have to be consciously diagnosed and responded to, see the story of the Staines air crash, 1972]. KBB is therefore what you have to turn to only when SBB or RBB are momentarily not up to the task at hand. This means that the goal of a given piece of KBB has to be "explicitly formulated" at the time it is needed, taking into account the nature of the problem and the overall aims of the subject. Examples of KBBs are problem solving and fault diagnosis. As for the nature of the information at this level, KBB is described as relying on symbols. These are defined as "abstract constructs related to and defined by a formal structure of relations and processes" (p261), and include language itself and mathematical equations.
Redrawn from a black and white original in Rasmussen (1983; Figure 1). This version Copyright © 2002, Derek J. Smith.
Slips, Lapses, and Mistakes
If interested in the error types associated with each of these three processing levels, see Section 2 of Unit HE2.
Rasmussen, J. (1983). Skills, rules, and knowledge: Signals, signs, and symbols, and other distinctions in human performance models. IEEE Transactions on Systems, Man, and Cybernetics, SMC-13(3): 257-266.
Reason, J. (1990). Human
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