How much would you pay for an envelope that had a 50% chance of containing $10 and a 50% chance of being empty?
Over time and in bulk, probably $4.99. But certainly not more than $5.
Here's where it gets interesting: how much extra would you pay for a plane that was guaranteed to be always on time, or a surgery that was always guaranteed to work? Suddenly, the same math that helped us value the envelope doesn't work so well. That's because we're often willing to pay a significant premium to avoid risk.
"Works every time" is a great promise to make to your boss. And it's the secret to Fedex's original success. Plenty of people send things by Fedex that don't need to get there superfast. They just need to get there for sure.
Doesn't work if you have to slip in the word 'almost' though.