[SystemSafety] Difference between software reliability and astrology

[Derek M Jones]

Phil,

> So my takeaway is that 1e-9 applies to all aircraft OF ONE TYPE and not all aircraft in the fleet. Keep in mind this was 
> written in 1988 when the skies were a lot less crowded.  So someone did some back of envelope math on flight hours per 
> day, number of aircraft of a popular type, and airframe lifetime and came up with this number.

The one TYPE argument does not change anything about the
worldwide incident rate.

Let's say there are 1,000 TYPEs, each containing 25 aircraft.

The incident rate for each type f 25 aircraft will be 1,000 times
lower than for the total of 25K aircraft.

However, adding up each of these 1,000 lower values gets us back
to the numbers calculated in previous emails.

If the number of aircraft increases by a factor of ten, then
a once in a 100 year event becomes a once in 10 year event.

No subtly needed.

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