Mixed CRs in 3.5

[Adept_Scholar]

There is a chart for mixed CRs in the 3.5 D.M. guide (pg. 49), yet this chart only tells how to define specific mixed CRs. For example, say I have a CR 12 and CR 4 creature together. What would their combined CR be? Would it be CR 10 (12+4=16 which is shown on the chart as 9+7=16=CR 10)?

[arkol]

If you're only looking for results and not the specific formula, I suggest you use this: http://www.d20srd.org/extras/d20encountercalculator/

[Porqueno]

A CR 12 creature on it's own is an appropriate challenge for a level 12 party, adding a CR 4 monster will not make it appropriate for a level 10 party. The tool I use for calculating encounter levels is this one http://www.d20srd.org/extras/d20encountercalculator/

just put in the number of monsters of each CR in the encounter and it will give you a total encounter difficulty. Note that often the total CR will not be a whole number, but this tool will automatically round it to the nearest whole number. So use caution.

[Douglas]

CR 12 + CR 4 would be very slightly above CR 12. By the book, it would take 8 CR 4 creatures to bump it to 13. Adding a creature never decreases CR, so that by itself should tell you that 10 would be horribly wrong.

CR scaling is roughly exponential. The primary principle is that 2 creatures of CR X is the same as 1 creature of CR X+2. A secondary principle to allow for scaling by +1 CR is that 1 creature of CR X and another of CR X-2 is the same as 1 creature of CR X+1. If you can't use some combination of those principles in some number of steps to get the combined assessment of extras up at least as far as 2 below the top CR in the encounter, then they're not enough to get even a single full point increase in overall CR.

Also, technically CR is only applicable to single creatures. When talking about the collective threat of a group of creatures, the correct term is Encounter Level, abbreviated EL. They're interchangeable and equivalent in the math, though.

Oh, and CR ratings below 1 get special treatment. For them, flip the rating upside down (so 1/3 becomes 3), and that's how many of them it takes to be equivalent to a CR 1.

[Adept_Scholar]

Using the link above, it says that one CR 12 creature + one CR 4 creature is "very difficult" for a party of 4 level 12 PCs, yet "easy" for 4 level 13 PCs. Why does it jump from "very difficult" to "easy" without a "challenging" inbetween?

[Jeraa]

"Easy" indicates an encounter below the parties level. "Very Difficult" is an encounter of 1 to 4 levels above the party.

A single CR 12 creature is "Challenging" for a level 12 party (it has a CR equal to the party level). Adding in a CR 4 creature would raise the CR, but not enough for a 13th level encounter. So lets call it a level 12.1 encounter.

A level 12.1 encounter is not "Challenging" for a level 12 party (As the encounter level is greater then the party level), so that bumps it up to "Very Difficult". However, a level 12.1 encounter is "Easy" for a level 13 group, as the encounter level is less then the parties average level.

Or at least thats my explanation.

But the CR system doesn't use fractional values. Adding a CR 4 creature to an encounter with a CR 12. If the party is strong enough to face a CR 12 creature, the CR 4 creature wouldn't be a threat. Its so far below the CR 12 creature is doesn't add anything to the encounter.

[TuggyNE]

Using the link above, it says that one CR 12 creature + one CR 4 creature is "very difficult" for a party of 4 level 12 PCs, yet "easy" for 4 level 13 PCs. Why does it jump from "very difficult" to "easy" without a "challenging" inbetween?

As mentioned below, it's essentially a rounding issue.


"Easy" indicates an encounter below the parties level. "Very Difficult" is an encounter of 1 to 4 levels above the party.

A single CR 12 creature is "Challenging" for a level 12 party (it has a CR equal to the party level). Adding in a CR 4 creature would raise the CR, but not enough for a 13th level encounter. So lets call it a level 12.1 encounter.

A level 12.1 encounter is not "Challenging" for a level 12 party (As the encounter level is greater then the party level), so that bumps it up to "Very Difficult". However, a level 12.1 encounter is "Easy" for a level 13 group, as the encounter level is less then the parties average level.

[Togo]

It's worth looking at what role you imagine the CR 4 creature to play. If it is a challenge in itself, like an orc with a sword, then it's not really relevant. If it's there to give a set of bonuses, or act as a speed bump, then it might be better to treat it as an enviromental hazard rather than a monster. After all, it's not there to be defeated, it's there to give the main encounter an advantage. In that case, just increase the main CR by 1.

[Doug Lampert]

There is a chart for mixed CRs in the 3.5 D.M. guide (pg. 49), yet this chart only tells how to define specific mixed CRs. For example, say I have a CR 12 and CR 4 creature together. What would their combined CR be? Would it be CR 10 (12+4=16 which is shown on the chart as 9+7=16=CR 10)?

There's nothing wrong with the other replies, but it's worth noticing that (baring roundoff and an odditiy at CR 1 in the XP tables), the XP and Encounter Level tables are based on the same underlying math.

You can get a good approximation to EL by simply adding up the XP values that the party will get for the monsters, and then looking for a CR that would give the party about that much XP.

So for your example, if the party is level 12 then they'd get 3600 XP for the CR 12 and 225 XP for the CR 4. A total of 3,825 XP. CR 13 is 4,800 XP so they're well under that but slightly above CR 12.

We can see from this that the CR 4 is almost (but not quite) irrelevant to this encounter's difficulty.

[Vladislav]

I would like to add that although the numbers may indicate that adding a smaller creature to an encounter doesn't change the CR, the tactical reality may differ. The low-level fighter may act as a speedbump, preventing direct access to the wizard and allowing him to cast a couple of spells unmolested, which tilts the odds dramatically ...

This is a good way for a DM to cheat and make encounters much more difficult than number-crunching may indicate :smallbiggrin:

[Adept_Scholar]

You can get a good approximation to EL by simply adding up the XP values that the party will get for the monsters, and then looking for a CR that would give the party about that much XP.



This makes sense Doug. Thanks for the replies all. :smallcool: