Bounded Accuracy: Playing Within the Lines

Last post we talked about system mastery and why I don’t really care for it as an intentional design goal.  Today I want to cover a design concept for Dungeons & Dragons 5E that I do like a lot: bounded accuracy.

So, what is it?  Well, one of the Wizards of the Coast developers wrote a a good but now defunct article explaining the idea a few years ago (Thanks again, WaybackMachine!).  In simple terms, bounded accuracy keeps the game consistent across character levels.  The philosophy focuses on static difficulty (i.e., skill check DCs don’t increase with level) and scaling based on inherent, limited character improvements.  When characters improve they don’t just get better and better at what they’re already good at, they get more versatile.

Why Is Bounded Accuracy Necessary?

Once again, we have to start with a bit of a history lesson to understand how revolutionary this idea really is.  Older D&D versions used a term called THAC0 – ‘to hit armor class 0″ – to express how difficult it was to hit a particular person or monster.  Frankly, THAC0 was an unintuitive mechanic.  Everything in the game except THAC0 increased in value as a character got better at it.  THAC0 was the only stat where lower numbers were better.  Often, THAC0 would drop into the negatives for truly challenging creatures.

In all of the later editions, the publishers abandoned the idea of THAC0 in favor of a much easier to understand positive ‘armor class’ (AC).  This made it a lot easier to compare two different creatures in terms of how hard they are to hit.  It’s got 16 AC?  That’s not so bad.  It’s got 25 AC?  Maaaybe we ought to come back later.

At this point in gaming history, everything worked on the same idea: bigger numbers are better.  However, the explosive growth of 3E, 3.5E, and Pathfinder additional source material had an unintended side effect.  With the right armor, feats, and magical accoutrement one could get to extremely high values of armor class.  The same held true for skills, to-hit bonuses, damage, everything.  I distinctly remember in one campaign our druid’s animal companion topped out at a staggering 27 AC at level 3!  As they grew in power, characters could eventually get to the point where they couldn’t be contested in their area of expertise.  They wouldn’t even need to roll dice to beat a non-specialist!

Huge numbers are a lot of fun sometimes but they have their issues.  For one, it becomes impossible to balance the game in any meaningful way.  Secondly, when writing the rules you have to make a lot of assumptions about how your intended audience will play the game.  If they stray outside of those assumptions, the game may become trivially difficult or impossibly challenging.

What Does Bounded Accuracy Do in Play?

So, here we are in ‘modern’ times and bounded accuracy has come along to save the day!  We can see the effect on characters right away.  At first level, most characters will have an AC between 10 and 20 at the very extremes.  Usually, they’ll end up somewhere around 14-16.  Additionally, everyone will have about +5 to their attack rolls, assuming they didn’t do anything too weird with their stats.  At the highest level of play, stats are capped at 20 (with a few exceptions) and the character’s proficiency bonus tops out at +6.  This means that, barring any sort of magical enhancement, a top-tier character isn’t that much ‘stronger’ than a new one.  Instead, as they gain levels characters get many new ways to address challenges.  In 5th edition, characters grow outwards as much as they do upwards, constantly increasing their repertoire of tricks.

These ‘limits’ ensure that the d20 roll always matters.  A low level character has a chance to hit even very tough monsters.  Likewise, weak creatures en masse can still pose a very real threat to high level characters.  Imagine my surprise when the D&D 5E Monster Manual came out and I saw the difference!  For a quick comparison, the adult red dragon (a classic staple) in 5E has an AC, attack roll bonus, and saves that average ten less than Pathfinder‘s equivalent creature.  When I saw that, the first thing I thought was, “That’s doable for a first level character.  Suicidal, yes, but actually doable!”

Once you realize that bounded accuracy was one of the major design elements of D&D 5E, you see it everywhere in the game.  The proficiency bonus that replaces scaling base attack bonus, saves, and skills.  Tougher limits on magical item enhancement bonuses.  How hard it is to increase your armor class past a certain point.  I would say that bounded accuracy touches on almost every aspect of the game.

Good Things Come from Good Design

So, why do I like bounded accuracy?  Well, there are a few different reasons.  To start with, pretty much everything listed in the referenced article is a good thing in my opinion.  Beyond that, there’s the following:

  • Having a concrete design philosophy and sticking to it helps ‘future proof’ 5th edition from the insane levels of power creep seen in earlier versions of Dungeons & Dragons
  • Knowing that bounded accuracy exists and the limits of the system allows 3rd party publishers to create content that plays nice with licensed D&D material
  • Putting classes on a more even playing field (I won’t go so far as to say ‘balanced’, but you know, close-ish, right?) and allowing players to contribute in situations they aren’t specialized in
  • Keeping the game simple with smaller numbers and less GM tweaking to match party and monster strength

Mainly, though, I like it because it’s a carefully considered design effort to focus on certain aspects of the game.  Dungeons & Dragons 5th edition was a major risk.  There were (and still are) a lot of people that preferred the old ways, with power levels that could literally scale to the heavens.  There were also a lot of people that hated the changes added in 4th edition.  If the design team had implemented it carelessly, bounded accuracy could have blown up in their face, big time.  Instead, they handled it with precision and style.  Nothing speaks to that more than the soaring popularity of the most recent addition to the Dungeons & Dragons legacy.