Shannon (unit)

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Shannon (abbreviated Sh ) is the unit for the information content of a message named after the American mathematician and founder of information theory, Claude Elwood Shannon .


A Shannon (1 Sh) is a unit of the amount of information in a message. The message is described by a set of characters and by the probabilities p i with which the individual characters appear in the message. By definition, such a message has the following informational content, measured in Shannon, of

The unit Sh results from the information theory established by Claude Shannon and corresponds to the entropy of the relevant news source and, in biometry, to the Shannon index of a population of different species.


  • The simplest example: A one-off message from the character set , where both characters occur with a probability of 0.5 each, has the information content .
  • If the character set consists of ten characters, for example , with all characters occurring again with the same probability, then the information content is . This can be interpreted in such a way that a 4-bit coding is definitely sufficient to transmit such messages (this is implemented, for example, by the BCD code ), but that there is still redundancy . With the BCD code, this redundancy manifests itself in such a way that only 10 of the 16 different characters that can be represented with this code actually occur.


Definitions of the units of decision content in information theory can be found in the international standards IEC 60027-3 and ISO 2382-16. In addition to the Shannon unit based on the logarithm for base 2, the Hartley based on the logarithm of ten (unit symbol: Hart) and the natural information unit based on the natural logarithm (unit symbol: nat) are explained.

"The attempt by the International Organization for Standardization (ISO) to replace the unit bit with the Shannon unit in 1975 [...] was not very successful in retrospect."

- Walter Umstätter


  • Johann Blieberger, Bernd Burgstaller, Gerhard Helge Schildt: Computer Science - Basics . 4th edition, Springer Verlag, Vienna 2002, ISBN 978-3-211-83710-8 .

Web links

Individual evidence

  1. ^ News for Documentation 49 (4), 1998, pp. 221–224.