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DMG82 says "When [calculating encounter difficulty multipliers], don't count any monsters whose challenge rating is significantly below the average challenge rating of the other monsters in the group."

How far below is "significantly below?" If I have a CR3 Mummy accompanied by CR1/2 Swarms of Beetles, should I count the beetles for purposes of difficulty multipliers? What about a CR3 Hobgoblin Captain supported by a CR1 Goblin Boss and some CR1/4 Goblins? Do I count the Boss and not the Goblins? Or does "significantly below" mean more like CR10 compared to CR1?

I know that in many cases I can figure this out by eyeballing it, testing, or using past experience, but what's a good mathematical rule of thumb?

I'm starting to run into this a lot when filling out encounters.

This is related to this question, but pulled into a more general case since that received responses relating to encounter design, not difficulty evaluation.

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The important point to remember here is that this decision only affects the multiplier on the encounter to determine how difficult it is. Therefore the points to note are:

  • This is a "rule of thumb" calculation in the first place. A "Hard" encounter is a very broad area and (despite what the table with its hard boundary suggests) it merges into "Moderate" at one end and "Deadly" at the other.
  • It has no impact on the XP the PCs gain in the encounter.
  • It is primarily a decision about encounter "pacing" or how many encounters the PCs have before needing to rest. You have way more direct control over this by e.g. bringing an encounter forward if the PCs are flush or deferring one if they are lean on resources.

Therefore it is primarily a judgement call that does not have huge implications. To illustrate consider the examples that you have given:

CR3 Mummy plus (3) CR 1/2 Swarms of Beetles

XP = 700 + 3 x 100 = 1000

Multiplier: 1 or 2 giving 1000 or 2000

CR3 Goblin plus CR1 Lieutenant plus (5) CR 1/4 Swarms of Beetles

XP = 700 + 200 + 5 x 50 = 1050

Multiplier: 1 or 1.5 or 2.5 giving 1050 or 1575 or 2625.

Against say 4 Level 3 PCs, in both cases the first multiplier rates this as "Hard" and any of the others are "Deadly" - what this says is that these are dangerous encounters; be careful. The low CR creatures will have a significant impact on the fight. So using them in the multiplier is reasonable because they low CR creatures will use up significantly more of the party's resources.

Against say 4 level 6 PCs, they are "Easy" to "Medium"; but the low CR creatures will probably be rendered hors de combat in Round 1 - their only contribution is to distract the PCs for that round and maybe burn a fireball. Not using the multiplier would be reasonable here because said fireball will probably be damaging the main protagonist as well so there is no additional drain on party resources.

CR10 and CR1

XP = 5900 + 200 = 6100

Multiplier: 1 or 1.5 giving 6100 or 9150.

This is a deadly encounter for the parties above with the first guy alone (and a probable TPK for the L3 guys).

For 4 level 10 PCs its "Medium" without the multiplier and "Hard" with it. However, given that the CR1 monster can probably be dropped with a Cantrip (3d10), Single attack or at most a 1st level spell (Sleep) it doesn't really add to the difficulty - he will distract 1 of the 4 PCs for part of 1 round. Ignore it.

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  • \$\begingroup\$ Are you sure about your math? Per DMG 82, level 6 characters have 900xp for Hard and 1400xp for Deadly, but that's per character. A party of 4 characters has an XP budget of 3600xp for a Hard encounter and 5600xp for a Deadly one. \$\endgroup\$ – kevin.matheny Mar 30 '15 at 3:58
  • \$\begingroup\$ Yep - forgot to multiply by the number of characters - I will fix that. \$\endgroup\$ – Dale M Mar 30 '15 at 5:42
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Since the guidelines from the DMG are purposefully vague, I will try to answer based on my own experience designing encounters.

First, all my examples assume a party of four 5th-level PCs. Their XP thresholds are:

\begin{array} \text{Difficulty} & \text{Easy} & \text{Medium} & \text{Hard} & \text{Deadly}\\ \hline \text{XP values} & 1,000 & 2,000 & 3,000 & 4,400\\ \end{array}

  • I playtested an encounter against 1 CR7 with 1 CR1/2 ally, with the help of a few Stack chatizens. Together, taking account the multiplier of 1.5, they have an adjusted XP budget of 4,500 (Deadly).
    The result was that it wasn't as hard as I had intended, at all. You can even read how the CR1/2 barely contributed to the fight from feedback by the playtesters. Ignoring the CR1/2 in the multiplier, the encounter would yield an adjusted XP of 3,000 (Hard), which felt about right, at the time. (We did this twice, the first time is here, and was even easier than the second)
    In conclusion, it would have been safe to ignore the CR1/2 creature in the multiplier.

  • I ran another encounter consisting of the following creatures:

    • 1 CR6 (2,300 XP) Necromancer
    • 1 CR3 (700) Skeleton Captain
    • 1 CR1/2 (200) Skeleton Marksman
    • 4 CR1/4 (200) Skeleton
      Taking into account the multiplier of 2.5, the encounter would have had a budgeted XP of 8,250- way past Deadly (the PCs had it coming, I swear).
      However, it didn't feel like the tough Deadly encounter I was expecting at all. The group managed to walk away, with no member even taking damage. (Full disclosure, one of the players was also using a notoriously overpowered wizard subclass, but I doubt it would have ended differently)
      If we ignore the CR3 and below creatures in the multiplier, it would have made a Hard encounter with an XP budget of 3,400, which sounds about right.

In conclusion

Based on the above, I would say any creature 3 steps below the highest CR creature in an encounter will not likely contribute to the battle in a meaningful way, so it is safe to ignore them when getting the encounter multiplier.

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