Originally Posted by
Efrem
Most errors involve pressing a key next to the correct one or transposing adjacent digits. Check digits can protect against 100% of those.
They can...
Originally Posted by
Efrem
But not on airline tickets. I didn't want to get technical, but they use the remainder modulo 7.
But they don't
If you transpose two digits with a difference of 7, the remainder mod 7 doesn't change.
Eg 81 = 18 = 4 (mod 7)
So it only protects against 84/90 = 93.3333...% of transpositions