Fault tolerance
Criteria

An example of a component that passes all the tests is a car's occupant restraint system. While the primary occupant restraint system is not normally thought of, it is gravity. If the vehicle rolls over or undergoes severe g-forces, then this primary method of occupant restraint may fail. Restraining the occupants during such an accident is absolutely critical to safety, so the first test is passed. Accidents causing occupant ejection were quite common before seat belts, so the second test is passed. The cost of a redundant restraint method like seat belts is quite low, both economically and in terms of weight and space, so the third test is passed. Therefore, adding seat belts to all vehicles is an excellent idea. Other "supplemental restraint systems", such as airbags, are more expensive and so pass that test by a smaller margin.
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