Universal Principles of Design, Revised and Updated

Redundancy

The use of more elements than necessary to maintain the performance of a system in the event of failure of one or more of the elements.

System failure is the failure of a system to achieve a goal—e.g., communicate a message, maintain a structural load, or maintain operation. It is inevitable that elements within a system will fail. It is not inevitable, however, that the system as a whole fails. Redundancy is the surest method of preventing system failure. There are four kinds of redundancy: diverse, homogenous, active, and passive.1

Diverse redundancy is the use of multiple elements of different types (e.g., use of text, audio, and video to present the same information). Diverse redundancy is resistant to a single cause of failure, ...