Rolling a d20 for Defense

[Wrathamon]

Wrathamon- now there's a twist!

Yah it is pretty interesting when the players kill themselves

they couldnt get mad at me for rolling well or fudging behind a screen... they did all teh rolls!

 

[JohnSnow]

I wouldn't do it in a system where you weren't using class bonus to defense.

If you are using a system where there's a meaningful class bonus to defense (such as Iron Heroes), it might make combat more dynamic. There'd be a more active feel to defense. On average, it wouldn't change, but it would leave open the possibility (however remote) of a low-level noob taking out the high-level character. Consider (I'll use Iron Heroes classes for the example):

A is a Man-at-Arms 12 (Atk: Longsword +18, Def: +17 (flat-footed +3))
B is a Man-at-Arms 1 (Atk: Longsword +5, Def: +6 (flat-footed +3))

I'm assuming each character has a longsword and heavy shield (+3 to defense), and has devoted a reasonable number of feats to those (given their levels). For simplicity, I've given A 3 levels of Weapon Focus & Shield Mastery (+3 to hit & to active defense), and devoted 2 of B's feats to acquiring 1 of each (+1 to hit and defense). That means A's devoted slightly about 1/2 his feats (6/12) to these two areas, whereas B's used 2 of his 3 starting feats. Which is about right, given their level differences. Assume both characters have Str 16 (+3) & Dex 13 (+1).

With static defense (10 + bonus), B's only chance to hit A is to roll a 20 (5%). A can't miss (100%).

With rolling for defense, B can hit A as long as he beats A's roll by 12. Which, while not a very good chance of success, is better than 5%. By contrast, A can hit B unless B beats his roll by 12. Which isn't very likely, but it's better than no chance at all.

Assuming they're both wearing full plate (DR 1d8), this fight could actually take a couple rounds.

Now, if A starts pulling out his other feats, B's probably still screwed. But that's as it should be. At least without the tricks, he's got a fighting chance.

 

[iwatt]

iwatt
Ok, I see your reference, but how does rolling a d20 vs using a static 10 make that situation worse and not better? It would seem to me having the large variable would make static bonuses less relavent, not more relavent. Or am I missing something?

Well, It's got to do with the probabilities of the opposed roll mechanic, whic I worked out a long time ago when I was working out the effect of the distance modifier to Spot and Listen.

Basically it coms down to this. Against a static DC, you have to roll good. Against a variable DC, you only have to roll better than the other guy. As your edge over your opponent improves, it's even more likely you'll succeed.

 

[ogre]

johnsnow- yeah, that was the feel I was looking for, and why I am considering a rolled defense. Thanks for working it out in detail.

iwatt- well, I guess I'll take your word for it, as I just can't seem to wrap my brain around it. I suppose I'd have to work out the probability stats to understand it. I can see what you mean though, about 'having to roll good' vs 'having to only roll better than your opponent' so I think I get the idea. Is it as simple as 'when you are at +10 more than your opponent'? or something like that? Meaning, when you reach a certain + above your opponent, the rolled defense becomes more predictable?

 

[Odhanan]

I have been contemplating initiating the 'roll your defense' mechanic into my PA game.
Meaning, instead of 'taking 10' on your defense all the time, you roll a d20 and add your defense bonus.
Has anyone tried this? Does it screw anything up? I personaly think players would find the fact that they can nullify an attack with a roll way more exciting than just waiting to see if they're hit. Am I overloooking anything?

Been doing that in our group for years now. It works great : you roll "1d20" instead of "10" for AC and Touch AC. The exception is when you are flat-footed, in which case it is still "10".

It is way more exciting when DM and players describe their actions and like dice rolls. It also opens a whole new range for feats and abilities that could affect AC rolls. It's all good for us at our game table.

It does affect game balance a bit. Namely, it makes Sneak Attack more effective in some situations, since there is now a clear difference between flat footed AC and the other types. That's far from game breaking, however.

Also, if you apply rules of fumbles and criticals on AC rolls, make sure to decide what effect they have before starting play. Fumbles on AC rolls at our game table trigger a chance of critical hit. Criticals on AC rolls may sunder the weapon, or have the weapon slip from the offender's grasp, depending on the situation and item. This actually makes combat more lethal on the long run.

Again, this will suit game groups who like random elements and dice rolls in their game. That will not be everyone's favorite option. Discuss it with the players first.

 

[iwatt]

iwatt- well, I guess I'll take your word for it, as I just can't seem to wrap my brain around it. I suppose I'd have to work out the probability stats to understand it. I can see what you mean though, about 'having to roll good' vs 'having to only roll better than your opponent' so I think I get the idea. Is it as simple as 'when you are at +10 more than your opponent'? or something like that? Meaning, when you reach a certain + above your opponent, the rolled defense becomes more predictable?

Well, to make it clearer.

When you roll a d20 v/s a static DC the probability distribution is uniform, and it's simple to find the following solution:

P(d20+X>=DC)----->P(d20>DC-X)

Now, the problem of solving for opposed rolls between player A with skill X and player B with skill Y is:

P(d20_A+X>d20_B+Y)----->P(d20_A+d20_B>Y-X)

Now, what we have here then is that we need the probabilities for a random variable consisting of the substraction of two uniforn variables. Unless you want to get into the math, you'll have to trust me that the distribution is triangular instead of uniform. This leads to the conclusion that a +5 difference means more in the opposed roll emchanic than in the statci DC system.

 

[ogre]

iwatt
wow, thanks for the more in depth formulas. Now, I understand math, but I'm no math wiz, so I can see what you mean, and will trust your conclusions. It is odd though, it just seems logical to me that the opposite would be true. Guess I'm not that logical hehe.
Very interesting, thanks for the input and to all those who have chimed in.

 

[GlassJaw]

Basically it coms down to this. Against a static DC, you have to roll good. Against a variable DC, you only have to roll better than the other guy. As your edge over your opponent improves, it's even more likely you'll succeed.

This is actually why I like opposed defense rolls. It gives the more skilled combatant a better chance of succeeding.

As I've said previously , I certainly wouldn't introduce this in a "standard" D&D game. I think a lot of people make the mistake of looking at certain variant rules in a vacuum. You just can't. The first thing I like to know when discussing rules like this is what else are you planning to include in your system. All rules compliment one another. When you change something, you are most likely going to the gameplay in another area.

For example, I would never use opposed defense rolls without action points. AP's reduce the randomness factor and sway the odds back towards the players. I also prefer opposed defense rolls and AP's in low-magic settings where the modifiers aren't going to vary as widely as they will in standard D&D. If the damage rules are fairly brutal and avoiding damage is paramount, opposed defense rolls will eventually be in favor of the PC's as their modifiers get better. However, you also have to consider their opponents and what they will be facing. Will most of their opponents be as powerful as they are or will they be facing mostly mooks? That also makes a big difference in the ruleset you use.

Think fluff first when deciding on variant rules - determine what you are trying to emulate first, then choose rules that support it. Don't just pick rules at random because they seem cool.

 

[Klaus]

I used to D&D, and it worked great. It removes the sense that the active character (i.e., the one rolling) failed, instead of the passive character succeeding (i.e., if you roll your attack and didn't hit, it was *you* who failed, not the other guy who succeeded; if you're rolling your Defence, it was *you* who failed to avoid the attack). Being opposed rolls, you might roll really well, but the attacker still hit, and vice versa.

I don't do any special outcomes for rolling Defense. Criticals and Auto-Misses are the purview or the attacker. The defender already has a special outcome by having a lowsy or awesome Defense rating.

Of course, this does open the field for Talents and Feats that are based on Defense rolls.

 

[iwatt]

This is actually why I like opposed defense rolls. It gives the more skilled combatant a better chance of succeeding.

Which is exactly the point I've been making. Just empasizing that they behave differently, and that that must be taken into consideration when deciding wether or not to use. I agree with you BTW, that the statistics of it make it a feature and not a bug. They do have the added effect of making secondary attacks even less probabable as well, but that's not really that relevant to the discussion (they already are unlikely to hit in the first place).