I find the name "Bounded Accuracy" confusing. Bounded - Ok limitations, sure, but Accuracy - maybe because it limits how often a give roll is successful for higher vs lower level PCs, but not every skill check is about being accurate. Many are about lies or deceit and deliberate inaccuracy. It sounds like it should just be called a power cap.
My question is: How can I relate the term Bounded Accuracy to its meaning?

up vote 30 down vote accepted

Accuracy refers to the percentage chance that a given attack will hit. An attack may have high accuracy or low accuracy, but both of these are a measurement of accuracy.

Common usage also applies the word accuracy to include things that we don't normally think of as a "hit", such as a skill check, but it makes more sense if we remember that the DC of a roll is what's called its "target number", as per PHB p.7:

3. Compare the total to the target number. If the total equals or exceeds the target number, the ability check, attack roll or saving throw is a success. Otherwise, it's a failure. The DM is usually the one who determines target numbers and tells players whether their checks, attack rolls, or saving throws succeed or fail.

The target number for an ability check or saving throw is called Difficulty Class (DC). The target number for an attack roll is called an Armor Class (AC).

CTWind's answer cites an informative 2012 article which appears to be the origin of the term "bounded accuracy", describing it initially in terms of attacks, then extending the concept orthogonally to other d20 rolls. Hence the reason why it's called "accuracy" even when it doesn't refer to attacks is that it's essentially the same mechanic as when it does refer to attacks.

In earlier editions of D&D, particularly third edition, numbers could get very high where high-level or highly-optimal characters with a lot of stacking bonuses were involved. A roll could be almost impossible for one character and almost guaranteed success for another.

Hence bounded accuracy, where the range of numbers it's possible for a character to roll is kept within tighter limits, or bounds. This shifts the focus of the game away from character optimization (by stacking small bonuses), and toward interacting with the fictional world by searching for in-situation ways to acquire Advantage.

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    I think in this context, "accuracy" is referring to the percentage chance that you can "hit" a particular target number, regardless of what the purpose of the roll is. – cpcodes Oct 24 at 0:51
  • I accepted this one because it tied the meaning of the terms to the origins of the phrase, and to the mechanics of die-rolling directly. Thanks! – MarkTO Oct 25 at 21:57

The earliest reference to 'bounded accuracy' as a D&D5e concept I've seen was in this (official D&D website) Legends and Lore article of the same name by Rodney Thompson (who, at the time, was a WOTC employee and is credited with Rules Development on the 5e PHB, amongst credit for other WOTC products).

While the whole thing is worth a read, here are some of the important bits:

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. [...] 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.

[...]

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.

My takeaway from this article is that the focus of the 'bounded accuracy' design philosophy seemed to be on not making hit chances and defenses scale by level to the point where level differences trump all else, and that new philosophy extended to how numbers scale with level throughout the entire game beyond just hit chance and AC, with the name simply sticking.

Ultimately, that name stuck because, so far as I'm aware, it's one that WOTC themselves penned to refer to that part of 5e's design.

  • Great answer, with some good background on where it came from. I wish i could accept more than one! – MarkTO Oct 25 at 21:56

In simple words, it's establishing upper limits to something that is hard to do. This applies to performance rating in industries, something not very seriously measured/quantified... but I will make it simpler for you to understand.

Consider 3 characters:

  • Character A: A typical commoner in every respect with average ability score with no proficiency
  • Character B: A level 1 Fighter. He would be able to add modifiers ( Ability and Proficiency 3 and 2 respectively)
  • Character C: Top fighter with highest modifier (5 and 6 respectively)

All three characters attempt to do something “hard”. They all need to hit 20. Character A rolls a 20 and succeeds. B’s roll is only 15, but with his +5 modifier he also succeeds. C only rolls a 9, but with his +11 modifier, he succeeds.

So, when we say "hard" thing - It's been hard for Character A, not so much for Character B. This is the kind of scenarios we see in our real life. Not every one is proficient in all their abilities. Some have a better chance of success than average people. Conversely, not everyone is as “average” as Character A. 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 could be tougher monsters that way because they can do more damage, more often, in more ways than characters with lower abilities.

A, B, and C can still have meaningful interactions with all these threats. So here, 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... Imagine encounters in groups and you'll understand it better.

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