In cybernetics, the number of distinguishable items (or distinguishable states of some item) is called the "variety'. So we may sum up by saying that the output variety must (at least) match the input variety for the system as a whole and for the input arrangement and the output arrangement considered separately. This is a vitally important application of Ashby's Law of Requisite Variety, which says that control can be obtained only if the variety of the controller (and in this case of all the parts of the controller) is at least as great as the variety of the situation to be controlled. This, like all profound statements of natural law, is perfectly obvious once it has been pointed out.

There is no great difficulty, however, in finding examples of attempted control systems which disobey this law quite flagrantly, and therefore do not succeed. From traffic control to the control of the national economy, this fallacy is apparent; indeed, this is one of the key problems of control in a firm. For management always hopes to devise systems that are simple and cheap, but often ends up by spending vast sums of money to inject requisite variety which should have been designed into the system in the first place.

Stanford Beer

<a href="/node/16083">Brain of the firm</a>