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Section
Appendix
4.3

Nines of Reliability

A system’s nines of reliability indicate the number of consecutive nines at the beginning of its percentage or decimal reliability. In the case of a risk of ruin, an additional nine of reliability means a tenfold increase in expected lifespan.

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Review Questions

In terms of the nines of reliability, calculate how much improvement we get by improving a system’s percentage reliability from 62% to 63%.

Answer:

k = -log(1-p)

For 62% reliability, p = 0.62, k = 0.420.

For 63% reliability, p = 0.63, k = 0.432.

So we get an improvement of 0.432 – 0.420 = 0.012 nines of reliability.

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In terms of the nines of reliability, calculate how much improvement we get by improving a system’s percentage reliability from 98% to 99%?

Answer:

For 98%, p = 0.98, k = 1.699.

For 99%, p = 0.99, k = 2.

So we get an improvement of 2 – 1.699 = 0.301 nines of reliability.

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In percentage terms, both situations above represent an improvement of 1%. In terms of the nines of reliability, which situation shows a more meaningful improvement, and how much more meaningful is it?

Answer:

The second situation shows a more meaningful improvement. It is about 25 times more meaningful than the first situation. (0.301/0.012)

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