The hartley (symbol Hart), also called a ban, 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, or shannon. One ban or hartley is the information content of an event if the probability of that event occurring is 1/10. It is therefore equal to the information contained in one decimal digit (or dit), assuming a priori equiprobability of each possible value.
As a bit corresponds to a binary digit, a ban corresponds to a decimal digit. A deciban is one tenth of a ban; the name is formed from ban by the SI prefix deci-.
One hartley corresponds to log2(10) bit = ln(10) nat, or approximately 3.322 Sh,[a] or 2.303 nat. A deciban is about 0.332 Sh.
Though not an SI unit, the hartley is part of the International System of Quantities, defined by International Standard IEC 80000-13 of the International Electrotechnical Commission. It is named after Ralph Hartley.