#bounded accuracy

"Why Bounded Accuracy?", by Justin Alexander

"Let’s start by talking about bounded accuracy. Endless ink has been spilt on this topic, but I think one of the clearest way to understand bounded accuracy — what it is, why it works the way it does, how it’s supposed to be used — is to look at the design lineage which created it.

To do that, we need to go back about twenty years to the development of the Epic Level Handbook for 3rd Edition. The concept was to extend play past 20th level, allowing players to continue leveling up their characters forever.

The big problem the designers faced was that different classes gained bonuses to core abilities — attacks, saving throws, etc. — at different rates, which meant that their values diverged over time. By 20th level, the highest and lowest bonuses had already diverged so much that the difference exceeded the range of the d20 roll. This meant that any AC or DC you set would either be an automatic success for some PCs or impossible for others.

The designers of the Epic Level Handbook tried jumping through a whole bunch of hoops to solve or ameliorate this problem, but largely failed. As a result, the Epic Level Handbook was a pretty flawed experience at a fundamental level (and its failure may have actually played a major role in Wizards of the Coast abandoning the OGL and the doom of 4th Edition, but that’s a tale for another time)."

[The Rogue notes: I think the big problem with 3.5 was that the breaking of the d20 roll (where the AC or DC you set could be auto-fail for some and auto-success for others) happened LONG before epic levels, if players made characters with different levels of optimisation. Which was sometimes a result of, well, studying, pouring through splatbooks and looking up combos on the internet, but other times it just happened, without any effort. Some classes had to jump through hoops to keep up with the rest, and that was bad.]

"On that note, fast forward to 4th Edition: The designers knew this was a problem. (Several of the designers had actually worked on the Epic Level Handbook.) They wanted to avoid this problem with the new edition.

Their solution was to level up everyone’s bonuses across the board: Classes would be strong at some things and weak at others, but the values wouldn’t diverge. This methodology was, furthermore, wedded to 4th Edition’s design ethos of “level up the whole world with the PCs” and more or less fundamental to its My Precious Encounter school of encounter design.

Fast forward again, this time to 5th Edition: The 4th Edition of the game had burned down, fell over, and then sank into the swamp, and 5th Edition’s mission was to win back the D&D players they had lost. The whole “level up the world” ethos was widely identified as one of the things people who hated 4th Edition hated about 4th Edition, so it had go.

Bounded accuracy was the solution. Importantly, bounded accuracy was about two things:

Controlling AC & DC so that the target numbers never become impossible for some of the PCs.

Controlling bonuses so that the results don’t become automatic successes for some of the PCs.

In other words, all of the results exist within that boundary. Hence, “bounded accuracy.”

If you go back to the original problem experienced in 3rd Edition (and which metastasized in the Epic Level Handbook), you can see how this solves the problem. It also avoids the 4th Edition problem where your numbers get bigger, but your results never actually improve because the numbers increase in lockstep: As long as the DCs remain consistently in bounds, the moderate increases to the PCs’ bonuses will see them succeed more often as they increase in level, resulting in high-level characters who feel (and are!) more effective than 1st level characters."

– Justin Alexander | The Alexandrian, September 2022

Commentary: Bounded Accuracy and Skills

This very well-written summary was part of an article roasting the 5e skill system, and specifically arguing that Expertise is bad because it breaks Bounded Accuracy, and Reliable Talent makes it worse. And with this, I disagree.

I think that Bounded Accuracy is excellent for combat's standard rolls: attack vs AC, and saving throw vs DC. That's when you need numbers that challenge the whole party: some characters may have a better chance than others, sure, but the d20 roll doesn't become irrelevant because this one is guaranteed to succeed and that one is doomed to fail.

But for otherwise interacting with the world, I actually don't think the numbers need to challenge the whole party. I think immersion and simulation (I like these!) are better served by making such challenges tricky. Occasionally they will be too easy for some, and/or too hard for others, depending on where the characters focused their training. And when that happens, it's up to the party to figure out ways to make up for it, to look for other, creative solutions rather than get stuck on a skill check that one or more of them are doomed to fail, and in the end to acknowledge that some tasks are suited for only some of them.

So maybe half the party auto-failing to scale that wall means they need to find another way in, or use their spells, or have the athletic ones climb up and throw down a knotted rope. That's good! It's a complication that requires a solution other than rolling a single check! Maybe only the Wizard (with 2024 rules) has a chance of making that extreme Arcana check about a long lost artifact. That's great! It makes sense and it's immersive, they should be the only one able to make it. And maybe, if your goal is to stealthily scout ahead, don't send forth the clanging armoured warriors, only send the sneaky rogues. That's fantastic! It's basic tactics! What's not to like?