In describing the redundancy of raw data, recall that the rate of a source of information is the average entropy per symbol. For memoryless sources, this is merely the entropy of each symbol, while, in the most general case of a stochastic process, it is
the limit, as n goes to infinity, of the joint entropy of the first n symbols divided by n. It is common in information theory to speak of the "rate" or "entropy" of a language. This is appropriate, for example, when the source of information is English prose. The rate of a memoryless source is simply , since by definition there is no interdependence of the successive messages of a memoryless source.
The absolute rate of a language or source is simply
the logarithm of the cardinality of the message space, or alphabet. (This formula is sometimes called the Hartley function.) This is the maximum possible rate of information that can be transmitted with that alphabet. (The logarithm should be taken to a base appropriate for the unit of measurement in use.) The absolute rate is equal to the actual rate if the source is memoryless and has a uniform distribution.
The absolute redundancy can then be defined as
the difference between the absolute rate and the rate.
The quantity is called the relative redundancy and gives the maximum possible data compression ratio, when expressed as the percentage by which a file size can be decreased. (When expressed as a ratio of original file size to compressed file size, the quantity gives the maximum compression ratio that can be achieved.) Complementary to the concept of relative redundancy is efficiency, defined as so that . A memoryless source with a uniform distribution has zero redundancy (and thus 100% efficiency), and cannot be compressed.
Redundancy of compressed data refers to the difference between the expected compressed data length of messages (or expected data rate ) and the entropy (or entropy rate ). (Here we assume the data is ergodic and stationary, e.g., a memoryless source.) Although the rate difference can be arbitrarily small as increased, the actual difference , cannot, although it can be theoretically upper-bounded by 1 in the case of finite-entropy memoryless sources.