Question about the maths behind attack roll and defense rolls
 

So I was leafing through the GURPS 4th ed. rules and looking at the opposed roll concept, which I liked. One thing stuck out at me.

Attack and defense are not opposed rolls. They're close, but the important issue of margin of success is missing. The way it is, Attacker rolls against her skill - failure means it fizzles, but success merely means Defender rolls against his skill. Defender does not apply a penalty to this roll based on Attacker's roll.

From a mathematical standpoint, what's the effect of this? If the designers had decided to make it an opposed roll, and applied Attacker's margin of success as a penalty to Defender's defense roll, how would the game be different?

The only thing I can think of is Defenders would fail more often - pretty much on par with the probability of standard opposed rolls as they currently stand. Did the designers remove the margin of success penalty to make the game less lethal? Has anybody applied the margin of success as a penalty, and how has that made your game run?

Re: Question about the maths behind attack roll and defense rolls
 

what it means is that if someone has 28 sword skill, they have a 95% chance to parry and can only be hit by critical strikes, if i understand correctly.

Re: Question about the maths behind attack roll and defense rolls
 

The current system has the deceptive attack mechanic to make it effectively into a contest. The difference is that the attack must commit to a penalty before hand. This is a significant disadvantage.

It also gives an relative advantage to attackers who know what the defender's skill is.

Re: Question about the maths behind attack roll and defense rolls
 

hadn't noticed that.

huh.

Re: Question about the maths behind attack roll and defense rolls
 

It's a key feature of the system. 3e lacked such a system by default, and suffered exactly the problem you pointed out.

The books, unfortunately, don't really explain how important it is.

Re: Question about the maths behind attack roll and defense rolls
 

yeah, it had been my intention to expand the crit numbers beyond 6 (for skill 16) to 7 and beyond so that two master swordsmen would chop each other to pieces with crits instead of endless parries.

now i'm thinking that won't be necessary.

Re: Question about the maths behind attack roll and defense rolls
 

Why would you expect two master swordsmen to rapidly dismember each other instead of to have a long, drawn-out duel?

Re: Question about the maths behind attack roll and defense rolls
 

When I first saw this, I thought of how a duel between two highly skilled individuals might last much longer than that between two poorly skilled individuals... in fact I believe I made a hit chance chart... I may still have it. In the long run, it didn't matter... deceptive attacks and feints put that twelve minute's effort down the tube... but I believe it's called... um... er... bagu...

Multipicitive Percentages?

Take the chance you have to make your attack roll, and multiply it by the chance you have to make your active defense. If I had my books on me, I could put a page number on where you'd find the odds, it's at the begining of the "Skills" chapter in the Basic Set Characters.

Apparetly I still have the chart though... if both characters have a skill 15, no enhacements to parry, and the attack goes unmodified... you have a 50% chance to hit, but if both characters have equal skill and combat reflexes (granting a +1 to active defense rolls), you have roughly a one in three chance to hit an unmodified attack until you get into the 18+ skill range.

Eh... at least I kind of thought that was interesting.

Edit: Come to think of it, Deceptive attacks bring the 1/3 chance to hit on equal skills up and beyond the 18+ skill range, if that's all you're doing with your attack.

Re: Question about the maths behind attack roll and defense rolls
 

Not quite: It would be the chance of a successful attack roll multiplied by the chance of a failed defense roll.

So assuming no other bonuses, no fancy maneuvers, and combatants with Skill 16 (Parry 11), each attack has a success rate of roughly 36.7875%.

If the attacker takes a -2/-1 Deceptive Attack (giving target rolls of 14 and 10), that improves to about 45.35%

A -4/-2 Deceptive Attack gives a slight additional improvement, to about 46.3%.

Edit: Incidentally, if you decrease each character's base skill by 1 (to 15 each), the odds of that unmodified attack hitting jump up to 47.7%, and a -2/-1 Deceptive Attack gives you 52.4%.

Re: Question about the maths behind attack roll and defense rolls
 

The concept is that the attack roll is answering the yes/no question of "Did you swing well enough to connect?" while the defense roll is answering the yes/no question of "Did you get out of the way/interpose an object in time?" The reason Quick Contest mechanics aren't used is because of choice. I'm not particularly good at combat - I might have Karate Sport at DX. Against someone who's decent, I can throw attacks that should connect all day, only to have them Parried or Dodged. I can increase my chances of scoring a hit by getting tricky - a quick fake-out before an attack, a particularly rapid attack, trying to hit that little opening I see, etc. In all cases wherein I do this, however, there's a trade-off - I have a higher chance (and I've actually noticed this) of missing outright, without need for my opponent to do anything to avoid the hit. This is what GURPS calls a Deceptive Attack*, which imposes -2 to hit for -1 to the enemy's defense. From the other side, the situation is fairly comparable - I can Parry or Dodge (retreating in both cases) their normal attacks fairly well, and they will frequently do a fake-out or quick attack in order to connect. The important thing here is that there is a risk, and the fighter must choose to engage in this risk in order to benefit from it. When you make an attack, you (in general) make the attack you were intending - you don't mess up and suddenly make it better.

That said, including Quick Contest like mechanics won't break the game, although they will result in more hits. The element of risk on the part of the attacker is lessened, as he needn't sacrifice his chances of hitting to decrease the chances of a successful defense. Do note, however, that Parry is based on half one's skill level - so it would be appropriate to half the attacker's Margin of Success to determine the penalty. If you don't like doing this, you could adopt the rules from T-bone's FEND (Fully ENabled Defenses) system, where Parry is Skill, rather than (Skill/2)+3, Dodge is Basic Move*2 (instead of Basic Move+3), defenses take a general -4 penalty, and penalties from Feints are doubled. In that, Deceptive Attacks trade off on a 1-for-1 basis - using Quick Contest mechanics, this simply means you are automatically going as deceptive as possible and still landing a hit.


*At my skill level, I'm probably doing something more akin to Telegraphic Attack normally, and then simply being less telegraphed when I do what I'm calling a Deceptive Attack. The math works out the same either way, however.

Re: Question about the maths behind attack roll and defense rolls
 

i wouldn't expect them to, but under my previous understanding of the rules, I would PREFER they chop each other up quickly as opposed to having a net 5% chance to hit per turn.

Re: Question about the maths behind attack roll and defense rolls
 

So, as I understand it, you want them to dismember each other because it's quicker and more convenient for you, as opposed to adhering to realistic expectations?

(I'm not saying there's anything wrong with wanting convenience, I'm just verifying what your position is.)

And in any case, even if you bump them up to Skill 26, the 5% hit chance is hyperbolic.

Skill 26 means Parry 16 (and what the heck, let's give them Combat Reflexes and Enhanced Parry 1, for a Parry of 18).

The attacker can take a -16/-8 Deceptive Attack, dropping both the attack and the parry to Effective Skill 10, and there would be a 25% chance of successfully striking the defender.

A -14/-7 deceptive attack: 27.8%

-12/-6: 24.4%

So the -14/-7 is the optimal point, giving you a 27.8% chance of success per turn.

If you adhere to the "one-second turn" idea, then that means that there will be about one strike per four seconds per swordsman. It's not unreasonable, then, that such an encounter might last 15 seconds or less.

That's also disregarding other attack options. If the attacker uses a Committed Attack (Determined), he can get up to 37.1% with a -16/-8 Deceptive Attack, but he'll be in a bad spot defending afterwards.

And then of course there are Feints (and Beats and Ruses), dozens of techniques, rapid strikes, etc.

Re: Question about the maths behind attack roll and defense rolls
 

Right... okay, I forgot the formula... ungh... I think, and I hope I got it right this time, it's...

(((Chance to hit)-(Crit Chance))*(Chance to fail Active defense))+(Chance to Crit)

And when you have combat reflexes, you have probably within 5% of one out of three chance to hit the target... sorry I got things backwards.

The idea is that two master swordsman attacking each other several times before first blood may seem unattractive to some... if it is, you can always fast-draw first if you havethe higher speed and slice the guy with a deceptive All-Out Attack (Double) and get him before he does anything... I mean that's what I'd do if I was put into the situation... unless he already has his blade ready, then I would throw something and run, hide behind the corner and get him when his back is turned... or something else delightfully cheesy.

If you want it to take longer, take enhanced block, a shield, and wear the heaviest armor you can afford... and this just skims the surface...

...and I'm way off topic by now.

Re: Question about the maths behind attack roll and defense rolls
 

@landwalker you are correct, i am willing to sacrifice realism for (in this case) game flow. but as i said, i didn't understand the significance of deceptive attack so this particular fudging won't be necessary when my game starts.

Re: Question about the maths behind attack roll and defense rolls
 

The 5% hit chance was before I drew his attention to Deceptive Attack.

Re: Question about the maths behind attack roll and defense rolls
 

This is what it means.

The first table has the percentages as they stand today. The second table has the percentages if you use the quick contest rules.

In quick contest, the only thing that matters is the difference between the stats that are compared, so the listed a margin is the difference between attack and defense. And it shows the percentages of how often the character with the higest stat will win.

Re: Question about the maths behind attack roll and defense rolls
 

Yup thats it but it gets more comlicated quickly!

I jsut finished my first few web pages on the subject (Check my sig). I even have a google spreadsheet dedicated to the cause.

(Chance of a Crit) + (Chance of a Hit thats not a Crit)(Chance of Failed Defense)

there is also the matter of how likely you are to penetrate armor. (The odds your DMG exceeds theri DR).

Its all under the GURPS and game Balance link.

Nymdok

Re: Question about the maths behind attack roll and defense rolls
 

This could be quite accurate though. Skill improves, but in GURPS your hit points don't. And as a fighter becomes more and more effective at doing high damage, the human biology doesn't keep pace with the ability to absorb damage (no matter what DnD and its 100+ HP characters would have you believe).

This may not apply to all fighting styles, but I'd say it's an accurate model for several. In fencing matches it's been my experience and observation that mid-level fencers give the most "entertaining" matches. The high-level fencers strike so quickly that one or the other has blown through to their target in a handful of seconds. (Granted, this is under competition-safety conditions where the round is over upon a successful hit - but I can only imagine the real-life swordfight experience wouldn't be much different. Once you start taking damage, you're into the vicious cycle of injuries hampering performance, leading to more injuries.)

Re: Question about the maths behind attack roll and defense rolls
 

When I've seen two master martial artists fight, I don't notice that bouts go on longer. If anything, they're shorter than novice v novice fights (the novices spend a lot of time moving around doing utterly ineffective things).

Turning my observations to GURPS, what I've seen is that good practitioners basically throw Deceptive attacks that bring their hit chance down to a net of 13-14 as a matter of course (after footing, which is almost always good, and hit location and use of Techniques are accounted for). You throw the bestest blow you can without flailing, and you do it every time it matters.

The only time you don't do this is when you're using lower skill than you have in order to teach...

Re: Question about the maths behind attack roll and defense rolls
 

If you're talking about Critical Hits that goes to 6 or less at Skill 16 or approx. 10%.

To use the numbers from Basic Critical hits go from 1.9% at 14 or less to 4.6% at 15 to 9.3% at 16. Skill 16 also drops Crit Fails from 17-18 to just 18.

So the chance of getting in that fight-ender more than double by going from 14 to 15 and double again from 15 to 16. The chance of spazzing out goes down by a factor of 4 too. That's actually a very sharp delineation in "real" Skill levels.

So fights between masters really might be shorter.

Re: Question about the maths behind attack roll and defense rolls
 

Quick Correction - Dodge is not based on Basic Move, but instead based on Basic Speed.

Re: Question about the maths behind attack roll and defense rolls
 

The biggest reasons why we didn't use Quick Contests for combat:

1. Not all penalties to attack rolls should make the attack in question easier to defend against. In reality, the trickiest moves with the biggest penalties are often the hardest ones to defend against. On a more basic level, why would aiming for someone's foot (-4) make it more likely that the target would parry with a sword in his hand?

2. Conversely, not all bonuses to attack rolls should make the attack in question harder to defend against. Lots of big bonuses come from utterly telegraphic, predictable attacks that sacrifice speed and technique for basic aim. Some come from having a familiar weapon (Weapon Bond) or a balanced one, and mostly just help your aim as well.

3. Not all defenses use the same metric. What's Dodge based on in a Quick Contest model: DX, HT, twice Basic Speed, something else? Everybody will have an opinion. And suppose that we say it's a skill . . . now everybody needs a skill to avoid a crummy Dodge, but those who learn that skill at high levels will never need to parry.

4. You still need defenses separate from attacks, because there are some attacks that don't roll to hit (like a big rock that you have to dive out from underneath, or a scything blade trap) against which you'll need to defend. Using full skill resurrects the issue of metric . . . is it actually as easy to react (roll vs. skill to defend) as to act (roll vs. skill to hit)? Reality says "no."

5. In evenly matched duels, Quick Contests suck away drama. Neither side will do anything fancy, because he needs his full skill to contest the other guy's full skill. Thus, master swordsmen just slug each other at skill 20 instead of trying disarms, stabs for the heart, etc., which is pretty much the opposite of dramatic.

All told, the current system does the job. The only chink it its armor in 3e was solved by Deceptive Attack in 4e.

Re: Question about the maths behind attack roll and defense rolls
 

Possibly because he was intending to extend the Crit Success beyond 6. A Crit on your attack allows no defence, therefore, higher Crit chances for Masters would mean shorter fights than for novices. Tempting.

Re: Question about the maths behind attack roll and defense rolls
 

I'm not sure whether longer or shorter fights best characterize matches between masters. "Master" has many meanings. In real life, masters tend to size each other up and then shoot in; one gets the upper hand due to a tiny difference in technical skill; and it's over in seconds. In movies, masters duel for five or six pages of script, sometimes more.

However, I'm also not sure why long is necessarily bad. Why shouldn't a life-or-death struggle use up a little more game time than other things? Flow is one thing; pacing is another. It's hard to pace a game where the most critical contests take less time than long stretches of less-critical development.

Re: Question about the maths behind attack roll and defense rolls
 

Is Decetive Attack in the core books, or was it added on in a supplement? (IIRC it comes from Martial Arts, but I'm not sure.)

Re: Question about the maths behind attack roll and defense rolls
 

See pp. B369-370, under Deceptive Attack. It's a core concept in the basic combat rules.

Re: Question about the maths behind attack roll and defense rolls
 

I believe the 3rd edition compendium offered various options. It included doing deceptive attacks functionally automatically (instead of a quick contest though maybe that was there too) where every 2 points of success over skill gave a -1 to defend. It had an option for increasing critical success with high skill as well.

Indeed I loved the 'official' sanction of the maneuver option in 4th edition. It fit right in and I wouldn't personally revert to 3rd edition experiments, though I suppose they could suit certain conceptions and game worlds.

Re: Question about the maths behind attack roll and defense rolls
 

I've had fights that took an entire session, and they were the highlight of the campaign. The problem isn't "long," it's "boring," and those two aren't necessarily the same, provided the rules of the game are sufficiently interesting. GURPS generally is.

Re: Question about the maths behind attack roll and defense rolls
 

Yeah, that was almost good enough, but it had a few undesirable effects:

1. It was passive, meaning that masterful warriors couldn't take the initiative and ensure that they would wipe out mooks. That took some of the fun away from being a master.

2. It was random, meaning that master-on-master encounters tended to be the "Who rolls the first critical success?" contests that they were so often criticized for being.

3. It was totally risk-free. There was no chance that you'd get too fast or too tricky for your own good, which is rather common in reality.

Consciously selecting the penalty and taking the attendant risks put a lot more control in the players' hands and leads to battles that are more dominated by tactics than by luck. Overall, it's more fun and dramatic for most players.

Re: Question about the maths behind attack roll and defense rolls
 

Our group independently invented the Deceptive Attack mechanic back in the day, and we went through similar iterations -- automatically taking the margin of success, versus pre-declaring an attack penalty, or divide by 2 versus divide by 3 (as an automatic divide by 2 is pretty powerful). We settled on pre-declared, divide by 2, as being the most fun.

I was all set to write up our nifty invention and send it to Pyramid when the new Pyramid showed up at my door, and someone had beat me to it...

Re: Question about the maths behind attack roll and defense rolls
 

True enough. Part of the problem is that players who come from games where the only results are "did damage" and "didn't do damage" get bored if nobody loses HP. Ironically, reducing fights to "Whose HP get depleted first?" is the most boring of all combat models. GURPS is nifty in that even during prolonged exchanges where nobody loses HP, there are thrills and chills when the fighters' choices lead to near things (like making that Parry roll of 9 exactly when somebody swings an axe for your neck), and plenty of nondamaging effects: feints, FP spent on extra effort, falls (esp. during kicks), dropped or broken weapons, ground lost to someone whose attacks demand retreats and who always steps after attacking, etc. Also, there are lots of choices; you don't just roll to hit over and over, but can pick targets and techniques, select what defenses you use, etc.

I think the easily bored HP-counters might be more engaged if they had tracks to show not just personal HP, but weapon and shield HP, FP lost to extra effort or chokes, current defense penalty vs. each foe thanks to footing and enemy feints, current defense penalty of each foe thanks to the same in the other direction, current movement point cost due to footing and posture, etc. Making one's enemies' tracks worse and/or one's own values better would be meaningful even if it wasn't just the HP track all the time. Lots of gamers seem not to appreciate the value of "he's kneeling, with -2 to attack and defend, and triple movement cost" or "he's against the wall and can no longer retreat for +3 to Dodge," and rarely seek alternative victories like pressing the foe so hard that he uses up all his FP on Feverish Defense and falls over, or just disarming him and using Intimidation.

Re: Question about the maths behind attack roll and defense rolls
 

I tried quick contests, to speed things up. It speeded them up alright - first strike wins. Nice for the PCs, until they met faster NPCs. Something to use for PCs to wipe out waves of mooks; perhaps use MoS as a measure of time.

Better to stick with the RAW. Although, if your combatants are a pair of unskilled neophytes using Average skill at default, doing something to shorten the combat is tempting, unless you actually want 20-40 rounds of miss-miss, miss-miss, miss-miss*, Hit!-dodged-miss, miss-miss, miss-miss, miss-miss, miss-Hit!-dodge fail-(d6-2?! 1HP injury). Then again, even skill 5 can give you a critical hit on your first roll and that could be the fight over.
*statistically, you should multiply the miss-miss bits by 6... Better yet, get them to use their brains (IQ check: evaluate for +3, AoA det+4, Mighty Blows...)

Re: Question about the maths behind attack roll and defense rolls
 

Though those effects tend to be either fight-ending or extremely transient. In general, the advantage of a hit point system is that it makes combat relatively predictable (you don't have people flattened by a single bad roll, and a GM can know that a specific fight is highly likely to be challenging but beatable) and means you can pretty much tell you're winning (or losing) before anything too dire has happened (and, if you're losing, perhaps break out your Special Tricks, or whatever).

This isn't realistic -- realistically, random bad luck gets people killed in combat all the time -- but IME players don't really want realism.

Re: Question about the maths behind attack roll and defense rolls
 

Well, maybe D&D players don't or something . . . it seems that lots of GURPS players do. Problems arise when a GM who likes GURPS and realism decides to run a game for players who mostly don't share his views. It cuts the other way, too: I can't really have fun in a game where there's no chance to win with a coup; few choices other than "attack," "strike harder but not so accurately," and "use Special Trick of the Encounter/Hour/Day"; and no active ability to evade harm when my enemy hits me. Or to be precise, I can't have fun with that in an RPG. It's fine for computer games; indeed, computer games can do that so much better than RPGs can that I don't see the point of that kind of game on a tabletop.

Re: Question about the maths behind attack roll and defense rolls
 

Well, yes, though there's a selection effort there; you're more likely to play GURPS if you like the way GURPS plays.

Side Note: I actually rather like GURPS combat for some genres, but it's a taste I don't really see my players matching. Then again, I think I've run more TPKs than the rest of the people I've played with put together.

Re: Question about the maths behind attack roll and defense rolls
 

If i recall correctly, "Deceptive Attack" was developed either here in the forum or on the GURPSNet-l Mailinglist under the name "Trade accuracy for effect" (TAFE) already before the Compendium Version :)

Re: Question about the maths behind attack roll and defense rolls
 

Oddly, I had more near TPK's in my only (~6 month) D&D 3.5 game then I've ever had in decades of GURPS.

Re: Question about the maths behind attack roll and defense rolls
 

It couldn't have been here (might have been the Pyramid fora, which existed before and then alongside these), since this forum didn't exist before the Compendium.

Re: Question about the maths behind attack roll and defense rolls
 

I didn't say I'd only run TPKs in GURPS. In fact, I'm not sure I've run one in GURPS. I just tend to run games that push PCs limits, and when you do that every so often they fail.

Re: Question about the maths behind attack roll and defense rolls
 

Also, if combat isn't risky, characters can't be heroic in risking their lives for that which they believe is right.

And that's a loss.

It can also easily lead to a lot of random fighting, because fighting isn't dangerous. There's no encouragement for the characters, especially the on-stage characters (the player characters) to pick and choose their fights. I don't like a lot of combat in RPGs, I'd rather see a few important and exciting fights, than a lot of unimportant and routine fights.

Re: Question about the maths behind attack roll and defense rolls
 

I used that in a 3e campaign. What I found was that, 1) it was very hard for a less-skilled opponent to ever land a blow, and conversely it was very hard for them to avoid blows, and 2) people never did anything that would lower their hit chance, ever, and that AoA for +4 to hit was way too low-risk unless your opponent had strong armour (because you would almost never miss, and your opponent would very likely not defend, and thus never get to strike back).

Essentially it takes all the options in GURPS' combat system and says "Don't bother - these are a waste of time.".

Re: Question about the maths behind attack roll and defense rolls
 

The game would be boring :)

I don't like simple opposed rolls for attacks and parries.

First of all, this mechanic tends to blur a poorly executed/failed attack and a good, but parried, attack.
The difference is quite important: in the latter case, the defender's weapon might break, or he might incur in penalties for repeated defenses.

Moreover, choosing to perform a Deceptive attack (and how much Deceptively) is now an important part of GURPS tactics. Fighters can "push the envelope" risking difficult attacks which impose a huge penalty, or they can fight more conservatively and hope their opponent's defense fails.
Having all attacks automatically "as Deceptive as possible" would eliminate choice and risk.

=

I'll also point out that, if you make Attack/Defense a single opposed roll, defenses become considerably more difficult. They become almost impossible, actually (two fighters of equal skill will rarely be able to defend against each other). So if you want to follow that road, you should probably raise defenses a bit.

Re: Question about the maths behind attack roll and defense rolls
 

What about Feint guys ? Doesnt that work for what you want ?

Re: Question about the maths behind attack roll and defense rolls
 

I misspoke when I said "Pyramid" earlier. The title of the periodical that accompanied GURPS in the late 80s that I had in mind was "Roleplayer". I checked through the ones I had handy, but I didn't find the article I seem to remember. So perhaps it was just Usenet. Though my Roleplayer #3 was missing... but I think that's a bit early for this sort of rule anyway.