Law of Requisite Variety

The Law of Requisite Variety is the best-known result of W. Ross Ashby’s An Introduction to Cybernetics (1956). It is often summarised as “only variety can absorb variety”.

Variety

Ashby defined variety as the number of distinct states a system can be in, or the number of distinct possibilities in a set. A light switch has a variety of two; a chess position has an enormous variety.

The law

Suppose a regulator is trying to keep some outcome stable in the face of disturbances. Each different disturbance needs a different response if it is to be cancelled. If the regulator has fewer possible responses than there are possible disturbances, some disturbances will get through. So the variety of outcomes can be reduced only by matching it with variety in the regulator. Ashby proved this formally and connected it with Claude Shannon’s theorem on the capacity of noisy channels.

In practice

There are two ways to cope with a variety mismatch. Attenuate the variety coming in, by filtering, standardising or simplifying (rules, forms, product ranges). Or amplify the variety of the regulator, by adding staff, tools, delegation or autonomy. Stafford Beer’s Viable System Model is largely a design for balancing these flows of variety between a management and the operations and environment it deals with.

The law explains why central planners, small moderation teams and rigid rulebooks are overwhelmed by complex environments, and why effective regulation usually relies on distributing control to those closest to the disturbance.

Further reading

• W. Ross Ashby, An Introduction to Cybernetics (1956), chapter 11
• Stafford Beer, Designing Freedom (1974)