International Encyclopedia of Systems and Cybernetics

1. Repetition of elements in a message or system which permits to circumvent transmission errors or functional failures.

2. A measure of how much knowledge about parts of the system allows to know something about other parts, due to known constraints between them (H. ATLAN, 1989)

3. "…the number and variety of internal subsystems which can perform the same function equally well" (W.D. GROSSMANN & K.E.F. WATT, 1992, p.10).

In relation to the first definition, it should be noted, in M. MARUYAMA's words that: "If a message can be compressed shorter by an efficient coding, then the amount of space saved by the compression is called redundancy" (1992, p.196).

This is the original meaning given to the term by C. SHANNON in his theory of communication. Redundancy here is used to compensate noise and to maintain reliability in the communication process. K. KRIPPENDORFF states: "English writing is estimated to be 50% redundant which account for the ability of native speakers to detect and correct typing errors" (1986, p.64). Redundancy is also useful for reconstruction of mutilated messages.

As to W.D. GROSSMANN and K.E.F. WATT they state: "Redundancy… provides reserves" (Ibid., p.8). In this case, failure of one circuit or channel can be compensated and reliable systems constructed from somewhat unreliable parts (von Neumann).

If redundancy, in ATLAN's opinion, reflects constraints, or the organization of the system, it is however obvious that an excessively redundant system (i.e. very constrained one) cannot have much functional meaning and, if totally redundant, none at all.

This is why "one finds also variety or diversity as a property typical of the existence of an organization, especially when complex adaptative behavior is exhibited. This property is in fact opposed to the first one (i.e. Redundancy) and it is a measure of the uncertainty or unexpectedness of the so-called information content of a system, in the formalism of information theory. As such it can be used to measure the complexity of a natural organization" (1989).

Thus, any complex system needs simultaneously variety and redundancy, which are in some sense complementary.

When the system ages, its redundancy diminishes, while its variety grows, up to a limit which is a characteristic of the system. The construction of variety uses up redundancy.