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 )
