So, as the other thread has questions about it... can anyone actually explain how CR works to me? I've tried to read various Rules Books, but it's all very jumbled up and unclear. Maybe someone here could help me out.

~SE++

It doesn't.

Seriously, look at Shadows or, oh god, anything in MMII and a lot of MMIII. Compare them to hags, or the Zombie template, or golems. It's lulzy.

Pretty much. CR is at best a rough guideline to help new GMs.

CR was intended to give a DM a guide as to how difficult a particular monster / NPC / challenge was in relation to an average party. If the CR was equal to the party's average level, it was supposed to be something they could handle but would drain about 20% of their resources - hit points, spell slots, consumable items, etc. The idea was that 5 CR-equal encounters in a day was supposed to be the extent of what they could do, if they expended almost all their resources.

There were a lot of problems, though.

What WotC considered an average party and what most of the community cosidered an average party were very different. Here was WotC's party (I need to ask you to just trust me on this one):

1. A rogue, built to be the eyes and ears of the group, find and remove traps, and sometimes get in an attack or two that dealt sneak attack
2. A tank, whether it was a fighter or paladin or ranger or whatever. They were the guy meant to rush into melee combat and swing a big ass sword, or have a longsword and shield (WotC considered those two equal - don't get me started). And when they did that, monsters or NPCs stayed there and engaged the tank. "Aggro" isn't just for MMOs...
3. A blaster, typically a wizard focused on using evocation spells but it could be a sorcerer or (later on) warlock, etc. The key here is they spend almost all their spells blasting and very little doing things like save or die, battlefield control, buffing, etc - you know, all the things wizards are good at.
4. Healer. Typically a cleric, but other classes like druid could fill in here if needed. The idea was that this character would help the tank, by standing there and alternating between doing an attack and casting an in-combat healing spell to keep the tank up and fighting.
5. If the party had a 5th member, it would be a support character. A bard, a ranger, a monk, etc.
6. If the party had a druid, he / she used wild shape to do things like turn into a hawk to scout, or a normal everyday animal to fight. There was never the intention of making this the be-all, end-all of melee combat like it was. That's why it was errata'ed so many times - nobody internal to WotC had any clue what wild shape was capable of, and so they reacted to community posts to build the next layer of errata for the ability. But a druid was never ever meant to replace the fighter by being a better fighter. It was meant to be a support role that could help scout, help heal, help in melee, etc.

There's a lot of problems there, obviously. The biggest is resources. WotC never, ever expected that the whole party would have flight, for example, and would sit in the air and use ranged attacks exclusively. That's why you see so many mid to high level monsters with no ability at all to deal damage or effects at range - WotC expected the fighter to stand there and trade blows with it.

the other big, obvious issue is the design of the system itself. It was intended to make a group of PCs have roughly 5 encounters a day (more if they fought under their avg party level, less if they went above). But... then they designed adventures - Dungeon magazine and otherwise - that threw them against 10, 15 encounters a day. So right off the bat, they're screwing up the intended resource management.

the other, less obvious, problem is that freelancers and in-house writers weren't supposed to consider source material other than core when writing stuff. (If the book was part of a campaign setting, the campaign setting's main book was also taken into consideration.) That's why you ended up with stuff like Divine Metamagic - when you put that into just core it's not nearly as broken as when you throw in other sources. (and this is another point that I'm going to ask you to just trust me on.)

There was more to it, of course. But really, it comes down to fatal flaws in the entire design - both the CR system itself as well as how they went about designing splat books and so forth.

Or the TL;DR version: WotC undermined the intended use of CR almost from the start, both with serious design flaws and with poor specs for writers to work with.

I'll expand on Fire's post a little.

In theory, a random monster with a given CR has, on average, a roughly 50% chance of killing a random equally-levelled player before getting killed itself (a good example of this is Hyena or Shark vs. Level 1 Fighter). This gives about a < 3% chance of a party member dying when that monster faces a group of four characters of a level equal to its CR.

So, that's what CR was designed as -- a creature with a CR of X was a typical encounter for 4 PC's of Level X. It wasn't applied very consistently, doesn't scale well (either horizontally or vertically), didn't consider how RPS strategies affected the outcome, and was not prepared for the level of CharOp and min-maxing that took place in 3.0.

And to WotC, PC's were supposed to look roughly equivalent to the NPC tables in the DMG, with slightly higher base stats.

But it also wasn't supposed be very exact; it was just a rough estimate. D&D 3.0 wasn't designed to be like a Starcraft, with Balance ex Machina designed as the core principle of the game. In that kind of game, the play experience hinges on encounters with precise and accurate challenges. 3.0 was designed as something more like SimEarth, akin to a set of rules to govern a rough simulation environment. Difficulty is up to the players, but the game is built foremost around giving rules for a world-environment to function in a consistent manner. So, the thought is, why do you need a really exact, accurate, and precise CR?

In any case, this is how that d20 Encounter Calculator works, in pseudo-code if you don't want to read 2 pages of JavaScript. It yields an "effective encounter level" (equivalent to a single monster with a CR of that value):

monsterarray monsters = monsters;

for (i=0; i++; i<monsters.length) {
monsters(i).PowerLevel = 2^(monsters(i).CR/2);
}

monsters.EffectiveLevel = 2 * log2 (sum(monsters().PowerLevel));


So, for each monster, you take 2 to the power of half its CR (i.e. CR 3 = 2^(3/2) = 2√2). Add up all of that together. Then take double the base-2 log of that sum.

Wow, that's exceedingly convoluted for a simple thing like trying to figure out how many kobolds to throw at people and how much EXP they should get for success. Thank you guys so much for helping to explain all this.

~SE++

Well, to be fair, that's an extrapolation of what it works out to be when you have mixed CR monsters in an encounter.

If you're looking to have all of them the same CR, it's pretty easy start with 1 creature. Every time you double the number of same-CR creatures, increase the CR by 2. So for example, you have one small air elemental. That's CR 1. Now you make that two; now it's CR 3, because you double. Make it 4, it's CR 5. And so on. That part is pretty easy. But has a few problems:

- The scaling at large numbers of creatures is pathetic. There is no way, for example, that 16 small air elementals are equivalent to a CR 9 creature.
- The formula only works smoothly if you have certain numbers of creatures (1, 2, 4, 8, 16, etc). If I want 5 small air elementals, the formula breaks.
- The formula also only works smooothly if all your creatures have the same CR. Wanna mix a small air elemental and a medium air elemental? Have fun guessing.

And so that's where the calculator comes in - it take the formula and the other notes in that section of the DMG and smooths it all out into a single formula. As Plane said, the real problem is that difficulty is really a function of the players but CR (and the related Encounter Level) try to use a static value to measure it.

Hey, just a quick thanks to GobO_Fire and Planeshaper for their contributions in this thread. Lot of information being thrown out there.

Fresh DM here and while I can draft story and build maps, I'm hazy on the details of figuring out how to challenge the group without making it un-fun.

I always err on the side of being too easy until I know for sure what my group is capable of.

Individual CRs aren't bad at 1st and 2nd level. As I said above, though, mixing and matching gets problematic quickly.

Normally — there is a "special case" rule that if the monster CR is less than 2, it bypasses the first equation (which I didn't write in my pseudo-code, not that it really matters at such low levels anyways).



But yeah, it still doesn't scale, regardless. 16 CR's of 1 most definitely does *not* have a 50% chance of killing an 8th level character in open terrain combat, nor using anywhere close to 20% of the resources of an 8th level party of 4, mostly because of the way that being able to hit scales with level. And I certainly can't think of any 11th level character that can't single-handedly decimate a group of 16 Lions or Wights, for instance.

So, a "complicated" equation that only roughly works. It works for a handful of monsters that are around the same CR. It's not a precision science; 3.X is very much a "do what you feel is necessary" kind of game — even in the DMG, there are all kinds of sub-sections of advice on making more or less challenging encounters as desired.


I want to point out that this is very different in DDN, where damage scales with level, and chance to hit scales at a much slower rate. In DDN, it will be much easier to get swarmed by low-level foes.

Use NPCs. Seriously, a level or few lower with bad classes like fighter and you can have an interesting encounter that's eminently reusable and easy to modify. Just be careful when using range or things like scout or lurk. And this way you avoid the risk of accidentally using the listed CR of a cat, ephemeral swarm, or whatever.

NPCs are a good choice, in large part because you can opt to optimize them at the same level as your party and maintain a decent - not great, but decent - balance.

That said, I prefer Pathfinder's NPC CR (character level minus one) to D&D's (equal to character level). But that's mostly fiddly bits...