WIZ - Reliability indexes - page 1
All ZOZ objects have a "Reliability Index" or "RI". It measures the probability that the object will not be lost (by crashes, fires, or world-ending cataclysms). Of course, this measure is only an estimate -- but it has its usefulness. It is defined as:
RI = -log10(probability of a failure in the next one hour).
It can also be loosely thought of as "number of zeros" when expressed as "1 in a thousand" (1000 = 3 zeros, RI=3), or "1 in a million" (6 zeros, RI=6).
It can also be thought of as the number of "9"s in the probability that something does NOT fail. EG, if the probability of failure (in the next hour) is 1 in a thousand, that is a 99.9% probabily of not failing, and that is 3 nines, RI=3.
Here is a small table of RIs and their corresponding probabilities:
Probability of failure Probability of not failing RI
---------------------------- -------------------------- --
1 in a hundred = 0.01 99% 2
1 in a thousand = 0.001 99.9% 3
1 in a million = 0.000001 99.9999% 6
1 in a billion = 0.000000001 99.9999999% 9
1 in a trillion = 0.000000000001 99.9999999999% 12
There is little point in an RI greater than 12. A trillion hours = 114 million years. If something fails after that, I don't think anybody will care.