The Law of Requisite Variety

<< previous page

Ashby’s Law of Requisite Variety was presented in the Introduction to this paper.  This law has special significance in the development of engineering control and communication systems but it is a general principle that applies to any system, whether economic, social, mechanical or biological.  It has practical relevance for organisations that need to survive and grow in uncertain, turbulent environments and it is central to the design of business management systems - it has been claimed that Ashby’s Law is as fundamental to the discipline of management as Newton’s Laws are to physics. [1]   However, this useful rule is often overlooked by management theory, possibly because it disproves the idea that ‘command and control’ methods - top-down, traditional hierarchies and the rigid imposition of rules and regulations - represent the best way to manage complex situations; it is no coincidence that the classic term for a regulator in mechanical engineering systems is a ‘governor’. 

Ashby’s Law was derived from mathematical analysis, but in plain language it is very simple and seems little more than common sense - control can only be obtained if the internal regulatory mechanism of a system is as diverse as the environment with which it interacts - see inset and diagrams on opposite page.  Note that the variety is requisite: if the control system is too complex the system will not operate efficiently; but if it lacks sufficient internal differentiation, it might not be able to cope with variable demand, or the system might fail entirely.  One interpretation of this rule is that adaptive, flexible systems which generate a large range of diverse options are better able to manage change than systems which are tightly optimised around a rigid set of initial conditions - in Ashby’s words: ‘only variety can destroy variety.’ 

Another reading of Ashby’s Law is that control is limited by the amount of information available to the controller and the way in which this information is processed and communicated.  This is because the effectiveness of a control system depends upon its ability to identify relevant disturbances outside the system and to convey an appropriate response - the right information, at the right time, and in the right place.  Both explanations are really different sides of the same coin: a complex system viewed in terms of the information it processes (as a communication device) will show a massive amount of variety; the same system viewed in terms of the values of its critical variables (as a control device) should show minimal variation.  As the pace of external change increases, improved communication channels and information processing are needed to retain control.

Ashby defined ‘variety’ in mathematical terms as the total number of possible states of a system.  However, this concept is not directly relevant to managers, partly because most of the hypothetical states of an organisation are unlikely to occur and partly because the size of the numbers involved will be vast.  A more practical definition of variety is the number of ‘distinctions’ which a person is able to make in a given context.  Any control system is dependent upon measurements to find differences in the system over time - setting limits on the scope and scale of the variables within the system which can be identified and captured by an observer.  The Law of Requisite Variety implies that the selection of measurements, as perceived by all relevant individuals, is a fundamental aspect of any management system.

next page >>