hartley

A unit of information, the amount of information inherent in the choice of one of 10 equally possible states (compare bit). It is equal to log210 bits, about 3.321928095 bits.

To see why this value makes sense: 1 bit can designate 1 of 2 possible states (0,1), 2 bits one of 4 possible states (00, 01, 10, 11), 3 bits 1 of 8 possible states (000, 001, 010, 011, 100, 101, 110, 111), and 4 bits 1 of 16 possible states. So the amount of information in designating 1 of 10 choices obviously must fall between 3 and 4 bits.

It is named for Ralph Vinton Lyon Hartley (1888 – 1970).

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