[SystemSafety] Difference between software reliability and astrology

[Derek M Jones]

Paul, Steve,

> “Can you (or anyone on the list) help me understand how the committee arrived at 10^-5, 10^-6, 10^-7, 10^-8 as targets?”
...
> (1) Probable failure conditions are those having a probability greater than on the order of 1 X 10^-5.
> 
> (2) Improbable failure conditions are those having a probability on the order of 1 X 10^-5 or less, but greater than on the order of 1 X 10^-9.
> 
> (3) Extremely improbable failure conditions are those having a probability on the order of 1 X 10^-9 or less.

Say 25K commercial aircraft
https://about.ch-aviation.com/blog/2022/06/30/june-2022-global-fleet-size-analysis-by-ch-aviation/

Assume they each fly 5 hours per day, 365 days per year (I'm sure
people on this list have much more accurate numbers)

1 in 10^-5 per hour equates to 1.25 failures every day
1 in 10^-6 per hour equates to 0.875 failures every week
1 in 10^-7 per hour equates to 1.125 failures every 90 days
1 in 10^-8 per hour equates to 0.456 failures every year
1 in 10^-9 per hour equates to 0.9125 failures every 20 years

-- 
Derek M. Jones           Evidence-based software engineering
blog:https://shape-of-code.com