Pro tip: the Evasion skill also works against trap damage. So if you fail to disarm the trap, even with Evasion 1, you have a 50% chance to not take any damage. With Evasion 3, you have a 90% chance to not take any damage.
So on that bear trap that you have a 80% chance to disarm. Let's say you try to disarm 100 of these traps. On average, 80 of the traps will be disarmed. 20 of them will fail to be disarmed. But you'll only take damage from roughly 2 out of the 100 traps when using Evasion 3.
Also, if you accidentally step on a trap and Evasion is up, that 50%-90% will very frequently save you. So if you know a trap is somewhere in a room, you can add a little safeguard by throwing Evasion up. This helps a lot near chests in the Minotaur lair. Basically, Evasion is the way to deal with traps.