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A ban, sometimes called a hartley (symbol Hart) or a dit (short for decimal digit), is a logarithmic unit which measures information or entropy, based on base 10 logarithms and powers of 10, rather than the powers of 2 and base 2 logarithms which define the bit. As a bit corresponds to a binary digit, so a ban is a decimal digit. A deciban is one tenth of a ban; the name is formed from ''ban'' by the SI prefix ''deci-''. One ban corresponds to log2(10) bit = ln(10) nat, or approximately 3.32 bit, or 2.30 nat. A deciban is about 0.33 bit. == History == The ''ban'' and the ''deciban'' were invented by Alan Turing with I. J. Good in 1940, to measure the amount of information that could be deduced by the codebreakers at Bletchley Park using the Banburismus procedure, towards determining each day's unknown setting of the German naval Enigma cipher machine. The name was inspired by the enormous sheets of card, printed in the town of Banbury about 30 miles away, that were used in the process. Jack Good argued that the sequential summation of ''decibans'' to build up a measure of the weight of evidence in favour of a hypothesis, is essentially Bayesian inference.〔 Donald A. Gillies, however, argued the ''ban'' is, in effect, the same as Karl Popper's measure of the severity of a test.〔 〕 The term ''hartley'' is after Ralph Hartley, who suggested this unit in 1928.〔Reza, Fazlollah M. ''An Introduction to Information Theory.'' New York: Dover, 1994. ISBN 0-486-68210-2.〕 The ban pre-dates Shannon's use of ''bit'' as a unit of information by at least eight years, and remains in use in the early 21st Century.〔(【引用サイトリンク】title=GCHQ boss: Crypto-genius Turing brought tech to British spooks )〕 In the International System of Quantities it is replaced by the hartley. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ban (unit)」の詳細全文を読む スポンサード リンク
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