For FAR 25 aircraft, "extremely improbable" is defined as a failure rate of no more than 1 per billion flight hours (1E-9), established by a quantitative safety assessment.
In General
To put this into perspective catastrophic failures (part 25) for all causes occur at rates 100 times more likely (1E-7).
I'm a lot less worried about the system causing the crash then I am the pilot's contribution