Originally Posted by
DutchessPDX
And you can still lose a paper boarding pass. I always laugh at these comments. What's the point? Your made-up scenario is somehow better than my made-up scenario? I can tell you my phone is A LOT less likely to run out of juice than I am to lose a piece of paper in my bag.
There is some probability that you will lose a paper BP. Let's call this A. There is some probability that your phone will die and not be able to reboot. Let's call this B. If we assume that A and B are independent (and there's no reason to believe that there's any connection between these two events), then P(A and B) = P(A) * P(B). Since all probabilities are numbers between 0 and 1, P(A and B) <= P(A) and P(A and B) <= P(B). If P(A) and P(B) are both not exactly equal to 0 and not exactly equal to 1, then the <= in both equations is <. Therefore, it's better to carry the paper BP with you because it cannot hurt.
Satisfied now?
https://en.wikipedia.org/wiki/Redundancy_(engineering)