Ashby’s Law of Requisite Variety
“if a system is to be stable the number of states of its control mechanism must be greater than or equal to the number of states in the system being controlled”.
Variety in this case refers to the unique elements in a set. For example, if we ignore the order of occurrence, the collection
[c, b, c, a, c, c, a, b, c, b, b, a]
which has twelve elements, has only three distinct elements— [a, b, c]. Such a set has a variety of three elements.
The conceptual graph in Figure 1 of the Ashby Space comes from Boisot and McKelvey. The managing system must, at least, match the variety of stimuli coming from the controlled system. Insufficient variety results in loss of control.
Thinking about our healthcare system, we need to not become trapped in the idea that fixed administrative procedures can prevent or eliminate variation that arises out of the environment itself. An example from aviation is that in flying, we adapt with the complex physical system called weather. We do not waste resources in a vain attempt to limit natural variations of the atmosphere. Eliminating variation in social and biologic complexity will be harder still and lethal if it were possible.