Relative redundancy: a more stringent performance guarantee for universal compression

Standard redundancy measures the excess number of bits needed to compress sequences of a given length. Instead, we consider relative redundancy that measures the excess number of bits for sequences of a given minimum description length. Low relative redundancy implies that number of bits needed to compress any sequence is essentially the lowest possible. We show that low relative redundancy implies low standard redundancy, that while block relative redundancy resembles block standard redundancy, sequential relative redundancy is twice its counterpart, and that common algorithms achieving standard redundancy have unbounded relative redundancy.