With sneaking you can pass monsters without fighting them.
Your chance for that is:
With skill sneaking:
Luck * 5% + Dexterity * 10% + 25%
Without:
Luck * 5% + Dexterity * 5%
In Bat + Bird + Catform you got 70%
Invisiblity gives you another + 40%
Minus the Awareness of the Monster
Maximum is 80%

Additionally sometimes monsters do not notice you and do not attack then.
The chance for this is:
With skill sneaking:
(Luck + Dexterity) * 2%
Without:
Luck + Dexterity