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\$– nitsua60Commented 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:
$$\begin{array}{c|c|} \text{d20 roll needed} & \text{Attackers per hit} \\ \hline \ 1-10 & 1 \\ \hline \ 11-16 & 2 \\ \hline \ 17 & 3 \\ \hline \ 18 & 4 \\ \hline \ 19 & 5 \\ \hline \ 20 & 10 \\ \hline \end{array}$$
With Disadvantage:
$$\begin{array}{c|c|} \text{d20 roll needed} & \text{Attackers per hit} \\ \hline \ 1-3 & 1 \\ \hline \ 4-8 & 2 \\ \hline \ 9-10 & 3 \\ \hline \ 11 & 4 \\ \hline \ 12 & 5 \\ \hline \ 13 & 6 \\ \hline \ 14 & 8 \\ \hline \ 15 & 11 \\ \hline \ 16 & 16 \\ \hline \ 17 & 25 \\ \hline \ 18 & 44 \\ \hline \ 19 & 100 \\ \hline \ 20 & 400 \\ \hline \end{array}$$
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.
