How should I modify the "Mob attacks" table (DMG 250) when the attacking mob has an advantage? Isn't a flat +5 bonus too generous?
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\$\begingroup\$ related: how does rolling two dice and taking the higher affect the average outcome? \$\endgroup\$nitsua60– nitsua602017-07-10 18:25:51 +00:00Commented Jul 10, 2017 at 18:25
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2 Answers
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The numbers in the DMG result from calculating the probability of a hit and then seeing how many attacks are needed to produce one hit on average. For example if a creature needs to roll 17 to hit, it has a 20% chance or 1 in 5. So if 5 creatures attack 1 will hit in average. If you make the same calculations with advantage (rounding 1/chance to hit) you get this scale:
| d20 roll needed | Attackers per hit |
|---|---|
| 1–10 | 1 |
| 11–16 | 2 |
| 17 | 3 |
| 18 | 4 |
| 19 | 5 |
| 20 | 10 |
With Disadvantage:
| d20 roll needed | Attackers per hit |
|---|---|
| 1–3 | 1 |
| 4–8 | 2 |
| 9–10 | 3 |
| 11 | 4 |
| 12 | 5 |
| 13 | 6 |
| 14 | 8 |
| 15 | 11 |
| 16 | 16 |
| 17 | 25 |
| 18 | 44 |
| 19 | 100 |
| 20 | 400 |
If you compare this with +5/-5 you will see that it the case of Advantage the flat bonus would be much more [ehem] advantageous for mobs with a low chance of hitting. Needing a 20 would become needing a 15. The DMG would then give a hit per 4 attackers where the full calculation shows you would need 10. Also if you apply -5 for Disadvantage, a mob that needs 16 or more to hit would have zero chance. This is not really a practical concern as 16 or more enemies can rarely surround a PC, but still a theoretical difference.
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Basically, I think it comes down to consider a mob as double their number for advantage, or half their number for disadvantage.
With this method though, in some cases in which a mob has advantage, the hits may exceed the total creature in the mob, so be aware and tune it down to a number of hits equivalent to the total of creatures in the mob.
