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REDUNDANCY
one minus the ratio of the actual uncertainty to
the maximum uncertainty. "This is the fraction of the structure
of the message which is determined not by the choice of the
sender, but rather by the accepted statistical rules governing
the choice of the symbols in question." (Shannon and Weaver,
1948, p. l3)
The variety in a channel that exceeds the amount of information actually transmitted. Its most common forms: (1) repetitive transmission of the same message over one channel, (2) duplication of channels, of which each could handle the transmission by itself, (3) restrictions on the use of characters or on the combinations of characters from an alphabet to form proper words or expressions (see language), (4) communicating something already known to its addressee. In the process of communication redundancy is essential to combat noise, to assure reliability and to maintain a communication channel. English writing is estimated to be 50% redundant which accounts for the ability of native speakers to detect and correct typing errors. Parity checks, which are common in communication within computers, enhance reliability but only at the expense of using additional channel capacity. The amount of information actually transmitted is not increased by this device. Similarly, Indian governments of the Mogul period are known to have used at least 3 parallel reporting channels to survey their provinces with some degree of reliability, notwithstanding the additional efforts. (Krippendorff)
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