The Success roll is based on effective skill rather then actual skill.
Because GURPS uses a bell curve it helps to understand the chances to get a certain roll.
- A roll of 3 or 4 is always a critical success.
- A roll of 5 is a critical success if your effective skill is 15+.
- A roll of 6 is a critical success if your effective skill is 16+.
- a critical success on an attack roll is a Critical Hit
- a critical success on an active defense results in a Critical Miss for the attacker
- A roll of 18 is always a critical failure.
- A roll of 17 is a critical failure if your effective skill is 15 or less; otherwise, it is an ordinary failure (in enchantment it is always a critical failure)
- Any roll of 10 greater than your effective skill is a critical failure.
- A critical failure on an attack is a Critical Miss
- A successful attack is also a critical miss if an active defense critically succeeds to thwart it
- A critical failure on a parry also functions like a critical miss
- Other active defenses can also critically fail:
- A critical failure on a block unreadies the shield
- A critical failure on a dodge results in a fall
Not all skill checks are created equal and difficulty modifiers are a way to simulate this.
- Automatic: +10 or wave roll entirely
- Trivial: +8 or +9
- Very Easy: +7 or +6
- Easy: +4 or +5
- Very Favorable: +3 or +2
- Favorable: +1
- Average: no modifier
- Unfavorable: -1
- Very Unfavorable: -2 or -3
- Hard: -4 or -5
- Very Hard: -6 or -7
- Dangerous: -8 or -9
- Impossible: -10 or no success possible
Extra Time (noncombat actions): 2x as long gives +1, 4x gives +2, 8x gives +3, 15x gives +4, and 30x gives +5.
Haste: -1 per 10% less time taken; 90% (taking 1/10 the prescribed time) max.
Per http://www.sjgames.com/gurps/faq/FAQ4-3.html#SS3.1.3 all rolls are impossible if target number is below 3, but are still rolled if attempter does not realize impossibility to determine if it is a normal failure or a critical failure. A roll of 3 can be a critical failure if rolling against a -7 skill.