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A chain: every part must work
Backups: any one is enough
Voting: k out of n must work
Enter your numbers, then press Calculate.
From the book, Chapter 19. Ten steps at 99% each: 90.4%; 50 steps 60.5%; 100 steps 36.6%. Two backups at 90%: 99%; three 99.9%. Two of three sensors at 95%: 99.275%. A shared cause that strikes 5% of the time makes losing both 5.95%, about six times as often as independent failures. For 99.99%: 4 backups at 90%, 14 at 50%. Five wedding suppliers at 95% each all turn up 77.4% of the time.
Assumes parts fail independently, except for the shared cause you enter. Everything stays in your browser: no sign-up, nothing stored or sent.