WIZ - Reliability indexes - page 2

What is nice about this scale is that RIs are generally additive.

For example, if the RI of a system is 3 (1 in a thousand), and there is another system with an RI of 3, then the probability of BOTH systems failing at the same time (within the same hour) is 1 in a thousand thousand or 1 in a million, or an RI = 3+3 = 6.

This only works when the two systems are far away or otherwise isolated.

I'd consider a object with an RI of 3 to be "really poor". But if we have four such objects, located far away from each other, their combined RI is 12 (3x4).

An RI above 12 is probably silly. There is probably a higher probability that a comet will come and destroy the earth.