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Bounded accuracy is the term that WotC uses to represent a role playing game design concept. It is not a “rule” and you won’t find it in the Player’s Handbook, but it is the foundational design philosophy behind the core of 5 Edition D&D.
The “accuracy” part of the term refers to how hard it is to do something. For combat, this relates directly to armor class and bonuses to your attack roll.
The “bounded” part of the term refers to establishing upper limits.
There is a maximum Ability Score of 20, a maximum Difficulty Class of 30, and a maximum Armor Class of 30. There is a maximum Ability bonus of +5 and a maximum Proficiency Bonus of +6 making a maximum total bonus of +11 (resulting in a maximum score of 30 on a roll of 19.)
Also, there is typically no more than +1 on magic items, with +3 being the cap and representing things of artifact power. The game makes no assumption that you have magical enhancement bonuses on your weapons and armor.
This is all about the Core Mechanic: To resolve an action roll a 20-sided die and add modifiers. If the result is greater than or equal to a target number then the action succeeds.
Regardless if this target number is a Difficulty Class (DC) or an Armor Class (AC), the concept is the same.
|DC-or-AC||Difficulty||To Break||Armor||To Hit|
|5||Very Easy||a glass bottle||an inanimate object|
|10||Easy||a wooden chair||No Armor||a badger|
|15||Medium||a simple door||Leather Armor*||a troll|
|20||Hard||a small chest||Plate Armor**||a dragon***|
|25||Very Hard||a treasure chest||a tarrasque|
|30||Nearly Impossible||a masonry wall(1 ft. thick)||a deity|
|*with shield and +2 Dex modifier **with shield ***Adult Red Dragon is AC 19|
To explain the effects of bounded accuracy on the game, it can be illustrative to compare its effect on three different characters.
Let’s start with a typical commoner. We’ll call him Fred. Fred is average in every respect. All of his ability scores are average (10) and Fred has no proficiencies or special skills. The table above was designed with Fred in mind. If any task is hard for Fred, it has a DC of 20. Fred adds no modifiers to his d20 roll when he attempts a task.
Our second character is Norman. Norman is a first level Fighter. The highest modifier Norman could add to his d20 roll would be about +5 (Ability +3, Proficiency +2).
Our third character is Conan. Conan is a 20th level Fighter. The highest modifier Conan could add to his d20 roll would be about +11 (Ability +5, Proficiency +6).
All three characters attempt to do something “hard”. They all need a 20. Fred rolls a 20 and succeeds. Norman’s roll is only 15, but with his +5 modifier he also succeeds. Conan only rolls a 9, but with his +11 modifier, he succeeds. So this “hard” thing is hard for Fred, not so hard for Norman and it is easy for Conan. Being normal PCs, Norman and Conan are better at some things than they are at others. They do not have maximum ability scores in all of their abilities, and they are not proficient at everything. At some tasks, they may not have a better chance of success than Fred does. Conversely, not all Non-Player Characters (NPCs) are as “average” as Fred. At some tasks, a NPC may have an ability score that is higher than a PC and a larger proficiency bonus. So most tasks within reach of specialist also fall within the ability of a lucky novice.
Higher level characters and tougher monsters are that way because they can do more damage, more often, in more ways than lower level characters.
If you are new to D&D, this may all seem obvious, and hardly worth more than a passing glance. However, this is a break from some earlier versions of the game. In some earlier versions, your PC’s “to hit” bonuses and Armor Class increased with each level and thus forced monster attacks/defenses to also increase with level. This resulted in lower level creatures being unable to have any possibility of hitting higher level PCs and visa-versa. This was done in the very reasonable goal of allowing higher level PCs to combat tougher monsters. D&D 5e accomplishes this goal, not by making tougher monsters harder to hit but by making them harder to defeat by giving them more hit points. So as PCs increase in level they do improve in their ability to hit higher armor classes (although at a much slower rate) but their ability to defeat tougher opponents comes mainly from their increased ability to inflict more damage when they do hit, and their increased capacity to survive stronger attacks due to their own increased number of hit points. So in this edition, characters can meaningfully interact with the same threats for most of their career, if they so choose. Lower level monsters will still be a threat at higher levels if they are encountered in larger numbers.This was described by Rodney Thompson in Legends & Lore (June 4th, 2012) on the Wizards of the Coast website. This is no longer available on their web site, so I quote from it here:
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.
This extends beyond simple attacks and damage. We also make the same assumptions about character ability modifiers and skill bonuses. Thus, our expected DCs do not scale automatically with level, and instead a DC is left to represent the fixed value of the difficulty of some task, not the difficulty of the task relative to level.
The link is back up on the Wizard’s site if you want to read Rodney Thompson’s comments in their entirety : Legends & Lore Archive | 6/4/2012