Converting D&D DC to GURPS Skill penalties

[kmunoz]

My gaming group, which can't meet as regularly as we'd like, has decided to do a series of one-shots from different systems. But then they decided that they'd rather stay with GURPS rules. So I'm picking up some published adventures from various systems and converting them over to GURPS.

Our first foray will be a D&D 3.5 dungeon crawl. (Dungeon Fantasy 1 & 2 came out just in the nick of time.) I'm not having trouble with the monsters, or any of the seriously crunchy conversion stuff. But I'm completely stumped on how to convert DC (difficulty class) to GURPS skill penalties. I want to do it with some semblance of mathematical accuracy rather than an arbitrary "DC 15 = easy and Skill-0 = easy." Unless that's truly accurate, of course.

It's one thing to talk about probabilities and die rolls... but DCs are open-ended. The die roll is added to skill ranks and attribute bonuses. Which presents a problem: in D&D the die roll is vastly more determinative than it is in GURPS. Say you have skill rank 1 in Open Locks. Before I started working this out I imagined this to be equal to, say, Lockpicking-DX in GURPS. But the average schmoe with Lockpicking willl have Lockpicking-10, the average schmoe with Open Locks will have Open Locks 1.

So I roll 3d6 vs. Lockpicking, and just under 50% of the time I will succeed against a crappy lock (no skill penalty). Same situation in D&D terms: crappy lock, say DC 15. 35% of the time I will succeed (because I only need a 14 or better, so 13 times out of 20 I will fail.)

But a simple DC 15 / GURPS no penalty lock is pretty much the only one that makes sense to me. Let's take it up to DC 25... that's a nice, moderately tough lock. Say skill penalty -6 in GURPS. Same skill rank/level. In GURPS I'd be rolling against Skill-DX-6, which for the average person would be 4. Possible... insanely hard, but possible.

Completely impossible for a D&D character with 1 skill rank. Can't "roll" higher than a 21. Needs at least 4 more skill ranks to make it at all doable. But DC 25 is a *common* difficulty for things like locks... it *assumes* characters will have skill ranks and modifiers totaling above 1.

So here's the question: what does that mean for GURPS? If a DC 25 lock *requires* at least a +5 modifier via ranks & attribute modifiers, what's the best way to translate that requirement over to GURPS skill level penalties? In GURPS terms you'd (grossly generalizing here) want to have enough skill to offset the penalty enough to make it a 50-50 shot (say, Lockpicking-16 or 17, to offset a -6). Should I therefore be looking at matching required skill ranks to required CP cost (to boost the skill to 50-50)?

Am I overthinking this? I'll bet I'm overthinking this.

[Gudiomen]

Originally Posted by kmunoz

Am I overthinking this? I'll bet I'm overthinking this.

You're overthinking this.

Mainly, DC levels from D&D and skill penalties/bonuses in GURPS work differently. The assumptions on what's "easy" and what's "hard" are different.
In D&D you need to be quite competent to atempt something even mildly hard, while in GURPS you can try with just about any dificulty, although you're likely to fail if you're incompetent (and might get hurt in the process).

Also, I wouldn't call an easy, regular, run-of-the-mill lock a +0 difficulty. Remember, you use skill rolls at flat score under "adventurous and stressful situations". So I'd call a +0 lock "average" or "challenging". While, an easy lock would be at a bonus. And a hard lock at a penalty. Use the dificulty levels sugested in Basic, they clear this up quite a bit.

I wouldn't try to establish a paralel between the success rate. D&D tolerates much lower success rates and still calls people skilled. Honestly, If I can only lockpick an easy lock half the time I'm not that good a locksmith.

[Maz]

I agree with Gudiomen.

It's much easier, and probably more accurate to simple make some estimation based on what seems right instead of going into math.

Personally I would simply give a +/-2 difficulty per DC:5, with DC:20=+0.
The reason I pick DC:20 as the default is because a typical D&D adventurer is about level 4-10. At the lower end you can have an effective skill of +10 if you have really focused in a skill, while at the upper end is its easy to have +10 in a secondary skill. At levels 6-7 most have +10 in a few of their class skills.

So:
DC:5 = +6 [even at default everyone could do it]
DC:10 = +4 [Everyone with just the most basis of training could do it]
DC:15 = +2 [And easy task for anyone with training, difficult for the untrained running on default]
DC:20 = +0 [Takes a skilled user to make it routinely, but anyone with training has a fair shot at it]
DC:25 = -2 [Difficult even for a skilled person]
DC:30 = -4 [Takes an expert and even he would have trouble]
DC:35 = -6 [Only the most skilled can do this and still they would have a higher chance of failure than success. Even the best of the best would not be guaranteed success]
...and so on.

IMO, this also fits pretty good if you compare the "Difficulty Class Examples" in the D&D DMG p.31, with the "Task Modifiers" in GURPS Basic p.345.

[Bruno]

The "back of the envelope" guideline I use with skill ranks is usually Skill Ranks = points in the skill, but that's really only a rough number and it's probably better just to keep in mind that someone with a really high effective skill in D&D should have a really high effective skill in GURPS too.

DCs, on the other hand, because of the differences in the dice system, probably can't be directly converted but there are guidelines you can work out.

At the low end things are roughly linear, but because D&D has a flat progression and GURPS has a bellcurve, I'd be wary of converting high DCs linearly...

a DC 5 is really hard to fail at. In GURPS and in D&D this kind of task usually isn't worth rolling for unless you're operating under a lot of situational penalties. Call it a +8 or even +10.

a DC 10 is something that someone with 10s in all his stats and completely untrained in has a 50/50 chance to succeed at. That's probably a +4ish to effective skill, offsetting a default penalty of -4.

a DC 15 is something someone with a slight aptitude (a 12 in the stat) and 4 skill ranks (a level one PC who's trained in it a bit) has a 50/50 chance to succeed at. I'd say roughly +2 to +0

DC 20 is hard for someone with a skill modifier of +5 - only a 25% chance of success. That's an effective skill of 8. A lightly trained person (skill 10-11) is looking at a -2 or -3, and skill 10 or 11 sounds about right for a 1st level character.

A character with +5 skill can only succeed at a DC 25 task if they roll a natural 20. 5% chance of success is roughly an effective skill of 5, so call it -5 to -6.

I suppose if you want a rough rule of thumb, every +5 DC roughly equals a -2 or -3 GURPS TDM...

but honestly, I think you'll be better served by looking at the described situation and just using the GURPS rules (possibly get your hands on a copy of Dungeon Fantasy 2, the first chapter is full of good dungeon delving stuff).

A basic house lock, by the way, is probably +5 to pick. Many folks will use this to offset a -3 to -5 in "time spent" penalties to jigger the lock really quickly. A skilled lock picker can open a basic house lock in about the time it takes for me to fumble for my key ring and open the door.

[Maz]

Originally Posted by Bruno

I suppose if you want a rough rule of thumb, every +5 DC roughly equals a -2 or -3 GURPS TDM...

Seems like we guestimated the same numbers :) Must mean they are right. I just doesn't like basing things on D&D level 1 characters because they are lowpowered compared even to a 50 pts mook. I think to have a fair comparison with a typical GURPS character, you need at least level 4.

[Rupert]

Originally Posted by Maz

Seems like we guestimated the same numbers :) Must mean they are right. I just doesn't like basing things on D&D level 1 characters because they are lowpowered compared even to a 50 pts mook. I think to have a fair comparison with a typical GURPS character, you need at least level 4.

It also fits with my assumption that you can half a D&D bonus/penalty to get a GURPS one (based on D&D's rule of thumb that a basic advantage or penalty is +-2, and GURPS' assumption that the same is a +-1). Thus, I'd say DC15 = +0, halve the difference from DC15, and round in whatever direction suits your mood.

[Kromm]

I'm not going to attempt to analyze it. Comparing DCs intended for a linear distribution (d20) to penalties intended for a bell curve (3d6) simply isn't going to work out nicely. In particular, the impact of modifiers will depend critically on where the chap rolling 3d6 happens to be on the skill curve.

But on that last point . . . I wouldn't assume skill 10. I think if you're even going to attempt an analysis, you should do something akin to "GURPS skill level = 12 + D&D skill ranks/5." Let's face it, in D&D, if somebody is going to bother will a skill starting out, he's going to make sure it's one that will be getting +1 to +3 to from an attribute at 12-16, or about +2 on average. That's like a score of 12 in GURPS. After that, it's hard to rate skill -- but since DC increases are generally given in 5-point chunks while GURPS tends to give great weight to each -1, 5:1 is as good as anything else.

[Gavynn]

Go back and review the Basic Set p.345-346. A +0 is for "adventuring" tasks, ike "Driving roll in a high speed car chase" which is a lot harder than "pick average lock" or somthing. I really wish we had more examples of skills with their TDM, but all we have is driving. There was an attempt to start that on this board somewhere, but it faltered.

[Kromm]

Do note that many skills come with standard modifiers! Look for colored paragraphs like this:

Modifiers: Some modifiers.

Also, don't overlook the fact that there are several sets of standard TDMs for vast swaths of skills: Comprehension Levels (p. B24), Culture (p. B23), Equipment Modifiers (p. B345), Familiarity (p. B169), Geographical and Temporal Scope (p. B176), New Inventions (p. B473), Physiology Modifiers (p. B181), Tech-Level Modifiers (p. B168), and of course all the combat modifiers on pp. B547-550. The "generic" TDMs on pp. B345-346 are more "instead of" than "as well as," although you can certainly use them to fine-tune.

[TGryph]

For a simple answer in my campaign, I took this information from the table of the D20 SRD:

Difficulty (DC)
Very easy (0)
Easy (5)
Average (10)
Tough (15)
Challenging (20)
Formidable (25)
Heroic (30)
Nearly impossible (40)

And compared it the "Task Difficulty" column on page 345-346 of the Gurps Campaigns book. So, that worked out (roughly) for me to..

DC 0 = +6 or +7
DC 5 = +4 or +5
DC 10 = +0
DC 15 = -2 or -3
DC 20 =-4 or -5
DC 25 = -6 or -7
DC 30 = -8 or -9
DC 40 = -10

Adding a pinch of good judgement, it has worked for me for quite a while.