Thursday, October 9, 2014

Revising Critical Hits and Misses in GURPS

Doug's work on critical hits and optimum skill levels caused me to think some more on a complaint I have with GURPS: I don't like the current system for critical hits and misses in combat.

The current mechanism is simple, but somewhat unsatisfying: a natural 3 or 4 on an attack roll is a critical hit, and the opponent gets no defense roll. A natural 17 or 18 on an attack roll is a critical miss, and something bad happens to the attacker. An attacker with an effective skill of 15 or better also gets a critical hit on a natural 5. An attacker with an effective skill of 16 also gets a critical hit on a 6 and doesn't critically miss on a 17, though she still misses on a 17.

There's about a 5% of rolling a 5 or less on 3 dice, and about a 10% chance of rolling a 6 or less. So going from effective skill 15 to 16 only slightly increases the chance of hitting, but it doubles the number of critical hits. Against an opponent with significant defenses, that can increase the chance of getting through the defenses by 5%-10% on its own. And attacking at skill 16 also reduces the chance of critical failing from 2% to 0.5%, which is either not much of a difference or a fairly huge difference. A rare event goes from happening one time in fifty to one time in two hundred.

The reason that all this is unsatisfactory is that it makes optimizing fairly simple: attack with skill 16. Going below 16 can slightly increase the chance of getting an attack through defenses, at the cost of giving up potential critical hits (and their beneficial effects such as extra damage or bonus armor penetration) and risking more critical misses. There are circumstances where going to skill 14 increases the chance of getting a hit through defenses slightly (going from 37% to 42% or so in the most favorable case) but most people feel the extra risk of critical misses isn't worth the slight gain in some circumstances.

I wish the optimization wasn't so simple, and there were more situations where having an effective skill ranging between 14 and 18 were valid but different. Different players could make different choices, depending on their preferences and the tactical situation, without feeling that they were making an inferior choice.

Proposed House Rule

My rule is a simple modification of the existing critical hit rules. An attacker with an effective skill of 15 or better also gets a critical hit on a natural 5. An attacker with an effective skill of 18 or better would also get a critical hit on a natural 6. An attacker with an effective skill of 14 or better wouldn't critically miss on a 17.

It's just a simple tweak to the current rules, but I think it opens up some space in play. A moderately skilled attacker can drop her effective skill to 14 to rack up penalties on the defense without increasing her risk of critical misses. Or if she wants to maximize her chances of critical hits, she can raise her effective skill to 18 and go crit fishing. Depending on the situation, she may even want to hold at 16, maximizing her chance of a hit without getting critical: useful when she wants to draw out a defense to set up another character or has an attack that penalizes the defender for successfully defending (such as a lightsaber or some other annihilating weapon).


  1. I made your mods in my spreadsheet. At higher skill levels, the choice between 14 and 16 has merit; for odd skill levels it's always 15. This does not take into account critical misses, which were not included in my own analysis either, but are a primary point of your post.

    I'm not sure what to do here honestly. The very sharp gradations in how uber crits are in skill regions of interest matter a lot in the analysis. I tried making crits fixed at 4 for skill less than 20, and 5 at 20+. That actually flattened things out remarkably.

  2. At least at first glance, it seems like this would just mean "optimize at 18, not 16." Is there a nuance I'm missing?

  3. 18 maximizes your critical hits, but you're paying for that with a few points less of deceptive attack. For a skill 20 attacker against a defense 10 or defense 12 foe, attacking at skill 18 has a 65% and a 42% chance of getting though defenses, respectively, while going down to a 14 ups the chances to 76% and 57%. That's more than 10% of your attacks succeeding, which is noticeable.

    This scheme makes crit-fishing much less optimal. If you're a beginning DF knight with skill-20 and enough strength to smash through an orc thug on a successful hit, you do better at effective skill-14 than you do at effective skill-18: the crits don't buy you anything, but the DA pulls their defense down from a 12 to 10, or a 10 to an 8.

    At higher skill levels, Doug's rule of thumb of "go for a 16" is fairly viable, but crit fishing is much less attractive and you're penalized much more for it than you are for going for a 14. Depending on the foe, crit-fishing might still be a useful strategy (if you need bonus damage to penetrate DR, for instance) but it's not the only viable strategy.

    1. A few points less or 1 point less? It seems to me that, yeah, for a session or two, you might be mathematically better off but how fast does that go away as your skill goes up?

      I guess it's just because I literally never calculate my percentage chance to penetrate a given defense with a given skill. I don't tell anyone the skill or defense of their foes, either, so they have to operate blind, so the only things you can control for are your crit numbers. You can't optimize easily around defenses when you don't know what they are.

      In any case, like I said on the forums, I'm not arguing "stay at 16" is the only viable strategy.