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-, even though the ban and deciban are not SI units.
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.
Usage as a unit of probability
The deciban is a particularly useful measure of information in odds ratios or weights of evidence. 10 decibans corresponds to an odds ratio of 10:1; 20 decibans to 100:1 odds, etc. According to I. J. Good, a change in a weight of evidence of 1 deciban (i.e., a change in an odds ratio from evens to about 5:4) is about as finely as humans can reasonably be expected to quantify their degree of belief in a hypothesis.
- Good, I.J. (1979). "Studies in the History of Probability and Statistics. XXXVII A. M. Turing's statistical work in World War II". Biometrika 66 (2): 393–396. doi:10.1093/biomet/66.2.393. MR 0548210.
- Gillies, Donald A. (1990). "The Turing-Good Weight of Evidence Function and Popper's Measure of the Severity of a Test". British Journal for the Philosophy of Science 41 (1): 143–146. doi:10.1093/bjps/41.1.143. JSTOR 688010. MR 055678.
- Hartley, R.V.L. (July 1928). "Transmission of Information". Bell System Technical Journal VII (3): 535–563. Retrieved 2008-03-27.
- Reza, Fazlollah M. An Introduction to Information Theory. New York: Dover, 1994. ISBN 0-486-68210-2.
- "GCHQ boss: Crypto-genius Turing brought tech to British spooks". Retrieved 2013-07-08.
- Good, I.J. (1985). "Weight of Evidence: A Brief Survey". Bayesian Statistics 2: 253. Retrieved 2012-12-13.
- David J. C. MacKay. Information Theory, Inference, and Learning Algorithms Cambridge: Cambridge University Press, 2003. ISBN 0-521-64298-1. This on-line textbook includes a chapter on the units of information content, and the game of Banburismus that the codebreakers played when cracking each day's Enigma codes.