The Benefits and Challenges of Bounded Accuracy D in 5th Edition

Bounded accuracy is a fundamental design philosophy underlying the mathematics used in the core rules for attack roll hit probability. It is unique in D&D history, in that it is one of the few times the developer was publicly vocal about their development standards, going so far as to even give it a name, and expressing this idea through official statements. This is a big departure from the typically secretive or silent R&D department for past editions.

Bounded Accuracy D

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The basic premise behind the bounded accuracy system is simple: we make no assumptions on the DM's side of the game that the player's attack and spell accuracy, or their defenses, increase as a result of gaining levels. Instead, we represent the difference in characters of various levels primarily through their hit points, the amount of damage they deal, and the various new abilities they have gained. Characters can fight tougher monsters not because they can finally hit them, but because their damage is sufficient to take a significant chunk out of the monster's hit points; likewise, the character can now stand up to a few hits from that monster without being killed easily, thanks to the character's increased hit points. Furthermore, gaining levels grants the characters new capabilities, which go much farther toward making your character feel different than simple numerical increases.

Now, note that I said that we make no assumptions on the DM's side of the game about increased accuracy and defenses. This does not mean that the players do not gain bonuses to accuracy and defenses. It does mean, however, that we do not need to make sure that characters advance on a set schedule, and we can let each class advance at its own appropriate pace. Thus, wizards don't have to gain a +10 bonus to weapon attack rolls just for reaching a higher level in order to keep participating; if wizards never gain an accuracy bonus, they can still contribute just fine to the ongoing play experience.

Getting better at something means actually getting better at something. Since target numbers (DCs for checks, AC, and so on) and monster accuracy don't scale with level, gaining a +1 bonus means you are actually 5% better at succeeding at that task, not simply hitting some basic competence level. When a fighter gets a +1 increase to his or her attack bonus, it means he or she hits monsters across the board 5% more often. This means that characters, as they gain levels, see a tangible increase in their competence, not just in being able to accomplish more amazing things, but also in how often they succeed at tasks they perform regularly.

Nonspecialized characters can more easily participate in many scenes. While it's true that increases in accuracy are real and tangible, it also means that characters can achieve a basic level of competence just through how players assign their ability bonuses. Although a character who gains a +6 bonus to checks made to hide might do so with incredible ease, the character with only a naked ability bonus still has a chance to participate. We want to use the system to make it so that specialized characters find tasks increasingly trivial, while other characters can still make attempts without feeling they are wasting their time.

Bounded accuracy makes it easier to DM and easier to adjudicate improvised scenes. After a short period of DMing, DMs should gain a clear sense of how to assign DCs to various tasks. If the DM knows that for most characters a DC of 15 is a mildly difficult check, then the DM starts to associate DC values with in-world difficulties. Thus, when it comes time to improvise, a link has been created between the difficulty of the challenge in the world (balancing as you run across this rickety bridge is pretty tough due to the breaking planks, especially if you're not a nimble character) and the target number. Since those target numbers don't change, the longer a DM runs his or her game, the easier it is going to be to set quick target numbers, improvise monster attack bonuses and AC, or determine just what kind of bonus a skilled NPC has to a particular check. The DM's understanding of how difficult tasks are ceases to be a moving target under a bounded accuracy system.

It opens up new possibilities of encounter and adventure design. A 1st-level character might not fight the black dragon plaguing the town in a face-to-face fight and expect to survive. But if they rally the town to their side, outfit the guards with bows and arrows, and whittle the dragon down with dozens of attacks instead of only four or five, the possibilities grow. With the bounded accuracy system, lower-level creatures banding together can erode a higher-level creature's hit points, which cuts both ways; now, fights involving hordes of orcs against the higher-level party can be threatening using only the basic orc stat block, and the city militia can still battle against the fire giants rampaging at the gates without having to inflate the statistics of the city guards to make that possible.

It is easier for players and DMs to understand the relative strength and difficulty of things. Under the bounded accuracy system, a DM can describe a hobgoblin wearing chainmail, and, no matter what the level of the characters, a player can reasonably guess that the hobgoblin's AC is around 15; the description of the world matches up to mechanical expectations, and eventually players will see chainmail, or leather armor, or plate mail in game and have an instinctive response to how tough things are. Likewise, a DM knows that he or she can reasonably expect players to understand the difficulty of things based purely on their in-world description, and so the DM can focus more on the details of the world rather than on setting player expectations.

It's good for verisimilitude. The bounded accuracy system lets us perpetually associate difficulty numbers with certain tasks based on what they are in the world, without the need to constantly escalate the story behind those tasks. For example, we can say that breaking down an iron-banded wooden door is a DC 17 check, and that can live in the game no matter what level the players are. There's no need to constantly escalate the in-world descriptions to match a growing DC; an iron-banded door is just as tough to break down at 20th level as it was at 1st, and it might still be a challenge for a party consisting of heroes without great Strength scores. There's no need to make it a solid adamantine door encrusted with ancient runes just to make it a moderate challenge for the high-level characters. Instead, we let that adamantine door encrusted with ancient runes have its own high DC as a reflection of its difficulty in the world. If players have the means of breaking down the super difficult adamantine door, it's because they pursued player options that make that so, and it is not simply a side effect of continuing to adventure.

They called this design philosophy bounded accuracy, because none of the designers were from the marketing department because it put boundaries on the numbers they were allowed to use when changing hit probability for monsters, by codifying total check bonus and AC limits for player content.

If you're going to be contributing content for the 5th edition of D&D, you need to be aware of the intended boundaries imposed by bounded accuracy for developers, as their intent is part of the precedent of 5th edition. Creating content which allows players to exceed the AC boundaries or get attack bonuses which greatly exceed any monster's AC will break the game. Creating monsters with unreachable AC scores will have a similar effect. Because the core mechanic is retained in 5e, and is still based on the same math as attack rolls, DCs for checks and saves generated by content also wind up being restricted to within the BA limits for the sheer practicality of actually being passable. Because BA alters the hit-frequency of attacks, if you are designing new attacks for the game, you need to understand that combat effectiveness at increased levels is based on making and landing more attacks, not scaling damage. While some things, like spells, can deal increased damage at higher levels, this is more of a function of the limited action economy surrounding other action types.

I am very curious about the community's opinion on bounded accuracy in 5E. Do you (or your DM) think that items which increase stats break the game? Are you limiting this in your campaings? Not allowing Headbands of Intellect, or Belts of Giant Strength, etc...

I have only been playing (and DMing) 5e for about 6 months, so to be honest, my brain's jury is still out regarding bounded accuracy -- frankly, it's still out regarding a lot of mechanisms in 5e. I suspect the way I will find out if I want to limit things like Belts of Giant Strength in hindsight -- i.e., after it is too late. I will have a better idea after I have a campaign or two and a couple of years under my belt. Of course, by the time my brain's jury comes back with a verdict on the mechanisms of 5e, 6e will be out and I'll be starting from scratch again.

Well, OK, but I like crunching numbers and looking at statistical properties of the game.... and I look at the balance from that perspective. Numerically, my personal jury is still deliberating about the whole 5e system, not just bounded accuracy.

Bounded accuracy makes it exceptionally difficult to award a lot of magic items that increase armor class, to hit or saves. In the past, there was a much wider range of Armor Class and To Hit for monsters. In the past, you would quickly progress to the point where a goblin would be no threat to you (5% change of hitting). Cut to 5E and and now those goblins on average have a (15% chance at hitting a level 20). Magic bonus capped at +3 for armor and most DM's are loathe to award that +3 save until level 18+. Why? 2ff7e9595c