One of the real world problems we face is that environmental influence can lead to errors. This problem becomes increasingly important as the relative size of the information systems is reduced. It will be a serious problem for a quantum system. It is already a problem in high speed memory storage, it is even an occasional problem in DNA copying, where frameshift errors usually result in cell death, but sometimes result in actual working mutations (like viruses). We need methods for detecting and correcting errors.
The opposite of compression is redundancy. If we use redundant digits carefully, we can detect and even correct errors which occur due to environmental influence (external noise). Copying errors occur in many forms: . Cells have, by nature, redundancy (single cells are rare).