Another issue which comes into play, is the probability of hitting the monster and how many rounds it takes to finally kill a monster. In general, a harder to hit monster can significantly drag on an encounter to more and more rounds.
Here's a model of this which is exactly solvable mathematically. The assumptions are:
- a target starts off with H "hit points"
- an attacker has probability p of hitting the target
- the attacker attacks the same target each round
- each time the attacker hits the target, 1 "hit point" of damage is done on the target
For this model, the average number of rounds (denoted A) it takes to kill the monster is A = H/p, with variance H(1-p)/(p^2).
Looking at this formula A = H/p, an attacker with a 50% probability (ie. p=0.50, or a roll of 11 or higher on a d20) of hitting a target monster with 2 "hit points" (ie. H=2), it takes on average around A = 2/0.50 = 4 rounds to kill the target monster. For an attacker with a 25% probability (ie. p=0.25, or a roll of 16 or over on a d20) of hitting the target monster with 2 "hit points", it takes on average around A = 2/0.25 = 8 rounds to kill the target monster.
The "hit points" of the above model can be thought of as "health units". Using my first elaboration of Jack99's example in a previous post, a monster's 3d8+9 damage roll has an average numerical damage of 22 hit points of damage, which can thought of as 1 "health unit" damage. For a target paladin/fighter/barbarian with 175 hit points, this can be thought of as the target having around 8 "health units". If a monster attacking a paladin target has a 25% chance of hitting the paladin, it would take an average of A = 8/0.25 = 32 rounds to kill the paladin. For a monster attacking the paladin with a 50% chance of hitting the paladin, it would take an average of A = 8/0.5 = 16 rounds to kill the paladin.
For my second elaboration of Jack99's example, the monster having a 3d8+31 damage roll has an average numerical damage of 44 hit points, which can be thought of as 1 "health unit" damage. For a target paladin/fighter/barbarian with 175 hit points, this can be thought of as the target having around 4 "health units". If a monster attacking a fighter target has a 25% chance of hitting the fighter, it would take an average of A = 4/0.25 = 16 rounds to kill the fighter. For a monster attacking the fighter with a 50% chance of hitting the fighter, it would take an average of A = 4/0.5 = 8 rounds to kill the fighter.