When calculating a monster's CR, how do I account for whether the monster deals all its damage in one attack or via several attacks?
Based on the DMG, it doesn't matter how the damage is distributed, in a single attack or multiple. Yet I find that a CR2 creature is far more threatening if it does 20 damage per hit, than two hits for 10 damage each.
Analyzing this over 3 rounds, let's say at 50% chance of hit, there's a 12.5% of the worst case scenario (hitting thrice for 60 damage) for the single attack, while for two attacks it's merely 1.6% (and 3% for 5 or 6 hits). In other words, the max damage per round for 97% of the cases would be 40.
Of course the average is balanced by the higher chance of not hitting at all, but it feels that the maximum is more indicative of the difficulty of an encounter.
It also seems that as the CR goes up, creatures generally have multiattack. Of course, this provides the flexibility to attack more creatures, but it also provides a more uniform distribution of damage.
How do I account for attack distribution when calculating CR?
I'd say that having one instead of two attacks is roughly equivalent to 20-50% damage increase (possibly reduced if the monster is meant to be encountered in groups and depending on the to hit and expected AC); but I haven't actually tried using this as a rule of thumb.