Talk of "AC:HP ratios" is fundamentally wrongheaded. Ratios are great for comparing two different stats that do the same thing, but AC and HP do two quite different things. These two stats function in ways that are simply not equivalent, so you can't create a meaningful equivalence ratio between them. AC reduces the
rate at which you take damage. HP increases the total amount of damage you can take without being healed.
In a game like Diablo II, or even to a much lesser extent, EQ up through Luclin or so, one could work out an equivalence ratio (though that ratio would be specific to a particular mob) because you aren't receiving a steady stream of heals. In Diablo II (or in classic EQ if you were soloing as a melee class, for instance), you more or less fight a mob(s) until it's dead, then heal to full. In pre-PoP EQ raiding and grouping, Complete Heal really was more or less "complete" and was the primary source of healing; again, you basically fought until you nearly ran out of HP and then healed to full (the difference between raiding and grouping being the rate at which this happened). In these two sorts of situations, decreasing the rate at which you take damage and increasing the total amount of damage you could take genuinely do have equivalent effects on your ability to survive a given encounter. Suppose you have 100 HP, and a mob hits you for 5 damage every second. You'll live for 20 seconds. If you increase the total amount of damage you can take by 20%, you'll have 120 HP, and you'll live for 25 seconds. If you decrease the rate at which you take damage by 20%, the mob will hit you for 4 damage every second, and you'll live for 25 seconds.
Modern EQ is a very different animal, though. Tanking and healing in EQ is not a matter of gradually chipping away at your total HP and then healing to full all at once. Instead, modern EQ is about keeping healing per second above damage per second (HPS > DPS). Healers don't wait for the tank to nearly deplete his HP and then heal him to full all at once; heals aren't big enough for that (and damage spikes might kill the tank, but hold that thought, I'll get to that later). Instead, healers are healing continually as the tank takes damage. If the tank is taking damage faster than the healers are healing it, the tank will die. Increasing the total amount of damage the tank can take will extend the period of attrition before the tank dies, but it will not stave off the inevitable death. Decreasing the rate at which the tank takes damage, however, will enable the healers to heal damage faster than the tank takes it, and the tank will live. Suppose you have 100 HP, a mob hits you for 25 damage every second, and a healer heals you for 20 damage every second. You'll live for 20 seconds. If you increase the total amount of damage you can take by 20%, you'll have 120 hp, and you'll live for 25 seconds. If you decrease the rate at which you take damage by 20%, you'll take 20 damage per second, and you'll live forever unless your healers run out of mana (or go AFK...).
Moreover, in situations (like a grind group) where the healer is already able to keep you alive continually, decreasing the rate at which you take damage will decrease the rate at which the healer must heal damage. Decreasing the rate at which the healer must heal damage will decrease the rate at which the healer must spend his mana. Decreasing the rate at which the healer must spend his mana will allow you to grind longer without needing to stop for a med break, or it will allow the healer to spend mana on other things (like helping you kill the mobs faster). Increasing the total damage you can take, however, has no impact whatsoever on these situations, because you're never using all of your capacity for taking damage anyway.
We can clearly see that AC and HP do two very different things, and, in modern EQ, the thing that AC does is simply much more useful.
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The above
is a bit of an oversimplification; there is one thing I've been ignoring in all this. In my examples, I assumed that the tank is taking damage at a constant rate (i.e. that the mob deals the same amount of damage every time). Obviously in real situations, this isn't the case. If our imagined tank is taking 25 damage per second, that's an average, and he isn't likely to be taking 25 damage
every second; in reality he's probably taking 20 damage one second, 30 the next, then 50 damage, then the mob misses a round entirely (0 damage), and so on. Spike damage is one of the reasons people often cite for taking HP over AC. And there was a time in EQ's distant past where there was some sense to this (to a point). You wanted to have more HP than the mob could deal in one max damage combat round. If a mob's max hit is 50 and he can attack four times per combat round, you'd want to have at least 200 hit points. At that time, mobs were not as dangerous as they are today, and keeping HPS > DPS was not so difficult, so tanks wanted to be prepared for the 'worst case scenario' even if it were less efficient in the long run. As such, tanks would gear for HP until they reached a point where a max damage round couldn't kill them, and beyond that point they would gear for AC (well, not exactly, because gearing for AC would inevitably yield 'enough' HP, but I'm being charitable to the pro-HP crowd in this example).
Again, though, modern EQ is a very different beast. For one, it's absolutely impossible for a tank to have enough HP to survive a max damage round. There's content that can literally deal over 200k damage in a single round. There are even group content named mobs that can deal about 100k damage in a round. So it simply is not possible to have enough HP to survive a max damage round; a tank
can't be geared for the worst possible scenario. Moreover, though, AC doesn't
just reduce average damage taken over a period of time. It also greatly reduces the likelihood of taking a large damage spike.
Here we need to take an aside to explain how NPC damage is calculated in EQ (this is different, and considerably simpler, from how player melee damage is calculated). A mob's damage is composed of two parts: Damage Bonus (DB) and Damage Interval (DI).
*1 Every mob in EQ has two distinct constants for these two values. The damage actually inflicted by a given hit is determined by the following formula:
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DMG = DB + DI(n)
where "n" is an integer between 1 and 20 (inclusive).
*2 In other words, when a mob lands a hit,
*3 a roll is made, and the mob deals damage equaling its Damage Bonus plus the number rolled times its Damage Interval. Suppose a mob has DB=100 and DI=50. The mob's minimum hit will be DB+DI(1)=100+50(1)=150. The mob's maximum hit will be DB+DI(20)=100+50(20)=1100.
Now that we have the terminology and the formulae established, we can fit this into the puzzle of how AC works and why it's valuable. We've already established that, in essence, what AC does is to reduce the rate at which you take damage, but we've thus far said nothing about
how it does that. The DI roll in the formula above isn't simply a random roll from 1 to 20. It is a weighted roll. Basically the mob's Attack value adds weights to the high end of the range and the player's AC value adds weights to the low end of the value. The higher the mob's attack, the more likely the mob is to score rolls close to 20. The higher the player's AC, the more likely the mob is to score rolls close to 1. Notice that this means debuffing mob Attack functions similarly to buffing player AC.
*4 Now here's the kicker: with high enough AC relative to the mob's Attack, a player can essentially skew the weights so far to the low end that the higher values for the DI roll simply never happen. We call this "DI obsoletion". Many tanks will reach a point where a mob will never score a roll over 17. I even managed to obsolete DI16 for the content I used for grinding experience during SoD. The way the formula works, if you parse a hit distribution for a given tank against a given mob, you'll have a massive cluster of values around the median. For AC-heavy tanks in current content, this cluster will usually centre around DI3 to DI6 depending on the difficulty of the particular mob. And, with sufficient AC, there will be a dearth of near-max hits. For AC-heavy tanks in current content, there will often be virtually no hits above DI17 or DI18 depending on the difficulty of the particular mob.
So, again, we clearly can see that what AC does is simply much more useful than what HP does. In the modern world, gearing for AC actually protects you from spike damage far better than gearing for HP.
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Note that all of this applies to anyone who takes melee damage, not just tanks. Whilst it's true that non-tanks receive lesser returns on AC than do tanks, that doesn't change the fact that AC and HP do fundamentally different things, and the thing that AC does is simply more valuable. Obviously non-tanks should be gearing foremost for their respective roles (i.e. hStr for melee DPS, mana for caster DPS, etc.), but that being equal, even non-tanks would be wise to focus on AC rather than HP. Melees are wise to, for instance, use Metal powersources rather than elemental. And the new AC/mana/hInt/hWis augs are the best thing since sliced bread for casters and priests.
If people are interested, I can get into further details about how AC works, but this post is already pretty technical, and further elaboration doesn't really serve the purpose of this thread, so for the sake of limiting my verbosity, I'll leave it at that for now.
Footnotes:
1. These terms are the conventions within the player community for discussing NPC damage, but the developers actually use very different (and, in my opinion, more logical) terms. What players refer to as "Damage Bonus" is instead called "Base Damage". What players refer to as "Damage Interval" is referred to as "Bonus Damage". And the D20 value that players don't name is referred to as "Damage Interval". The developers are by and large aware of the different terminology the playerbase uses, but it's something to keep in the back of your mind when reading developer comments on the subject. The different terminological conventions are a product of the playerbase having figured out the damage formulae on our own in isolation from the folks who created said formulae.
2. Yes, EQ is, at it's heart, a D20 game. D&D fans everywhere rejoice.
3. Misses are an entirely separate calculation. You can't mitigate a hit to zero and turn it into a miss. First the mob must pass a roll to land a hit, and only then does this come into play.
4. The formulae for Attack is not simply the opposite of the formula for AC, however; the economy of scale for attack is smaller than for AC. The result is that debuffing X Attack has a bigger impact than buffing X AC.