What is the fundamental difference between 'redundancy' and 'diversity' as reliability strategies?

The fundamental difference between 'redundancy' and 'diversity' as reliability strategies lies in how they address potential failures. Redundancy involves using multiple identical or similar components or systems to perform the same function. If one component fails, the other component(s) take over, ensuring that the function is still performed. For example, having two identical pumps in parallel, either of which can supply the required flow, is a form of redundancy. The weakness of redundancy is that all redundant components are susceptible to the same common-cause failures, such as design flaws, manufacturing defects, or environmental factors. Diversity, on the other hand, involves using different types of components or systems to perform the same function. These different components are based on different principles or technologies, making them less susceptible to common-cause failures. For example, using a different type of sensor to measure the same variable or using a completely different method for achieving the same safety function are examples of diversity. While redundancy provides protection against random failures, diversity provides protection against systematic failures and common-cause failures. Therefore, redundancy provides backup through identical components, while diversity provides backup through fundamentally different components or methods.