Thus, I thought I'd take the opportunity to explain it, because there are many people who don't seem to understand exactly how mob damage output works. Therefore, this is a lengthy teaching post. It will demonstrate how damage values are figured, how AC, Shielding, and HP work towards a tank's effectiveness, and why non-tanks bidding on shielding augments are shortsighted.
I've gathered information regarding this for quite awhile, because I hoard information like cigarettes in jail. The Steel Warrior site has the best collection of information; SK.org also has some significant studies. If you'd like to look them up, you can easily use Google.
Btw, you can skip the math if it's too boring; parts II and III should be comprehensive.
I. THE EQUATIONS
Every mob has two damage values that are used to calculate how much they hit you for on a hit. These two values are: Damage Bonus (DB) and Damage Interval (DI). Every mob has these two values.
When the mob lands a hit on you, a number is generated from 1-20 (random d20). This number is multiplied times the DI, the result of which is added to the DB. This can be expressed mathematically as:
H = DB *(1.00-S) + (N*DI)
S represents the amount of shielding you have, up to a maximum/cap of 35% shielding. This is where a lot of confusion comes in, because people have an improper understanding of the value of shielding.
As the equation shows, shielding only modifies the DB portion of damage. It shaves off a fixed % of the bonus damage before it gets added to the DI result.
Removing shielding for a second for clarity, there are thus 20 discrete values for damage that may result, ranging from:
Minimum Damage = DB + 1*DI
Maximum Damage = DB + 20*DI
N, which is the random 1-20 number, is affected by your AC and class mitigation.
You can determine all of these numbers simply by watching your logs and damage intake. Let the mob hit you, and collect data, and you can find a range of hits, and extrapolate from there using the formula above.
For warriors, the formula is slightly different. With zero shielding:
Warrior innate mitigation = DB + 0.95*(1-20)*DI
Warrior defensive discipline = DB + 0.5*(1-20)*DI
Examples of how the formula works:
Mob_A: DB=50, DI=100. Shielding = 0
For non-warriors, minimum hit is 50 * (1.00) + (1*100), or 150.
Hit values are 150, 250, 350, 450, 550, 650, 750. . .and maximum is 2050 for non-warriors.
For warriors, minimum hit is 50 * (.95) + (1*100), or 145.
Hit values are 145, 240, 335, 430, 525, 620, 715. . .and maximum is 1950 for wars.
Under defensive, minimum hit is 100 and maximum is 1050.
Here, there's a big difference between the maximum hit under Defensive and without it. Further the range of possible hits (maximum hit-minimum hit) when not under Defensive is quite high making healing needs a bit unpredictable.
Mob_B - DB=1800, DI=10, Shielding = 0
Minimum hit is 1810 and maximum is 2000 for non-wars
Minimum hit is 1809 and maximum is 1990 for wars
Under Defensive, minimum hit is 1805 and maximum is 1900
There is very little difference between the maximum hit under Defensive and without it. Further the range of possible hits (maximum hit-minimum hit) is nearly identical with or without Defensive.
Thus, if a is mob hitting for a wide range of values, the chances are that it's DI is high and you will benefit from using Defensive. If two values are pretty close and it's consistently hitting for very similar values, the DB is doing most of the damage and DI is likely fairly low - use Evasive.
Now, DB is typically high on, you guessed it, boss mobs. This is where shielding comes in. Let's look at our two examples again, as warriors.
Mob_A: DB=50, DI=100. Shielding = 20
Minimum hit is 135 and maximum hit is 1940. Thus, on this low DB mob, we see a reduction of 10 hp per hit.
Mob_B - DB=1800, DI=10, Shielding = 20
Minimum hit is 1449 and maximum is 1630. On this high DB mob, we see a reduction of 260 per hit.
A high AC reduces the N value. We refer to this N*DI value as DI1 (min), DI2, DI3, up to DI20 (max). Thus, a 2500 AC tank might fall into the DI3 range a lot, or the DI4 range a lot.
For the sake of comparison, an average Tacvi named has a DB = 500 and DI = 250. This means its range is from DI1 of 750 to DI20 of 5500 for any class, or 738-5250 for warriors.
II. WHAT IS MOST IMPORTANT?
Shielding does not have a singular universal value. Some mobs have more of their damage coming from DB and some have more coming from DI. On a mob with DB 1 and DI 400 (which doesn't seem to exist; this is only illustrative) shielding will do absolutely nothing. On a mob with DB 1,000 and DI 10, shielding is invaluable. In general, your meanest mobs have more damage coming from DB, because this results in the minimum hit being higher. This means that in the roughest situations--such as tanking boss mobs--shielding is going to do the most good.
I parsed myself when I tanked Queen Pyrilonis, and found I was mitigating her into DI4-5 frequently. That's very good. A tank with lesser AC, say 2100, might mitigate into the DI8-9 range a lot. That's arbitrary, but it is a healthy example of the difference in damage intake, and why a high AC is arguably more important than having a lot of hit points.
Most people erroneously use hp as the primary judge of tank viability. While to a point, it's sensible because at the high end, high AC and high hp go hand-in-hand on items, for mid-level tanks, there's a wide range of itemization. An MPG group leader asking your warrior twink what his HP are, without asking for his AC, is an idiot.
What people sometimes fail to realize is that AC makes your HP more effective. As an example, if a mob has an average hit on you for 800 (say, a tank that goes all HP in MPG) and you have 10500 hp, you can absorb 14 hits. If the mob has an average hit on you for 400 (a tank that goes all AC in MPG) and you have 9000 hp, you can absorb 23 hits--or tank roughly 70% better.
1000 hp difference means very little if AC and shielding are equivalent between 2 tanks. On a mob with DI = 500 and DB = 250, they will be able to take less than 1 more hit on average (median is 2994, mean varies with attack but basically a war would need 2000+ hp difference to take even one more HIT).
That being said, true effectiveness is a balance of both--mitigating tremendous amounts of damage, but having enough hp to withstand high DI damage spikes (inevitable) and damage that does not come from mitigation, namely procs and AE's. If you have really nice AC but lack the hp to handle, say, General Reparm's 3kdd proc, you better hope your wizards have a lot of peridots. Indeed, on our own raids, when do we see a lot of tank deaths? When AEs hit.
One rounding is generally not a concern for tanks, especially with discs (defensive for warriors, mini-defensive for knights). What kills tanks is streaks of bad rounds and auxiliary damage. We tank mobs that hit upwards of 5k--for example, King Gelidran--and look how rarely one rounding occurs, almost never when heals are in motion. Usually, it happens on tank switches, or when heals aren't in swing.
AC and shielding combine to be much more effective at preventing spikes and reducing the amount of healing necessary. A high shielding/AC tank with 13.2k hp only has to be hit for 66% of KG's max damage in a round to be one-rounded, but because of his mitigation, he doesn't. Someone with lower AC, but 15k hitpoints, would be taking double, or possibly triple the amount of damage spikes--giving a much, much higher risk of death and needing much higher replenishment resources.
The value of AC is that it works independently of a healer; as opposed to hitpoints, it's not dependent upon constant refreshment in limited, hardcapped amounts to perform its function. Since hitpoints are dependent upon replenishment, they are typically dependent upon another person for their effectiveness (healers)--and frankly, other peoples' decisions are the worst form of mitigation.
Shielding is therefore better and better the more AC/mitigation you have, because proportionately more of the damage you are taking is coming from the DB.
As stated, shielding's worth values greatly. Aneuks in Txevu, for example, at 1000/1 (DB/DI), would reduce the mob's damage by nearly 35%. Who do you want tanking them? Having high ac does almost nothing on those mobs because their hits do not vary in any meaningful way due to their amazingly low DI. Here, you survive primarily due to shielding and hitpoints.
Comparably, GoD yard trash (100/50), hitting me at an average DI5 would mean an average hit of around 350. This would reduce to 315 with max shielding, or decrease damage intake by 10%. Mobs in GoD generally have low DB's though, so that's an extreme example.
III. ON SOFTCAPS
There's often confusion regarding the term "softcap" because it refers to two separate concepts.
(1) There exists a hardcoded cap that is based upon class, and is relatively low. Devs state that a median 65 warrior is above his class softcap; extrapolated, we can surmise that all classes at the high end have reached their hardcoded cap. In the Velious era, there was only this hardcoded cap; that was it. After a certain point, which differed for each class, the benefit of more AC didn't just diminish - it dropped to nothing.
Just before PoP, the system was changed from a hardcoded cap to a soft cap. Now, you get a percentage increase based upon the your amount over that soft cap. Shields increase both your total ac *and* your soft cap, making them more effective than any other item with equal AC. 100 AC on a shield is much more valuable than 100 ac on a breastplate. The benefit increases as the AC on the shield increases - it isn't a static bonus. This is why knights that tank with 2handers are not smart tanks. Because of how the shield affects mitigation, you are better tanking with a Time 1hander and a crappy Basalt Bulwark from Plane of Fire than by using your leet DoN or Anguish 2hander.
Additionally, your mitigation AA's, level, and class also affect the cap and the percentage return over softcap you receive; that is, how much of an increase in survivability does X amount of AC procide your class, and at what point does your AC completely overcome a mob's ATK value?
Classes mitigate differently, and plate tanks--warrior, SK, and paladin--have the highest mitigation, with the largest return over softcap for AC. This means that smaller raises in AC affect the hit distribution positively so that the mob hits for lower values. For example, a tank will notice a difference between 2100 and 2300ac. An int caster won't notice a difference between 1000 and 1700, despite the relatively huge ac increase (700) compared to the tank (200).
(2) This leads to the fact that your mitigation also varies depending on what mob you are facing. For instance, as a knight, go to Rifts and parse yourself at 2k AC. Then, parse yourself on the same mob at 3k AC. The difference is large, with benefits continuing to show as your AC increases.
Now, go to Sol B and parse yourself at 2k and 3k AC. There won't be much difference at all in value distribution, mins/maxes, avg. DI multiplier, etc., because you are so far over the mob's attack that the extra AC doesn't matter.
Because of this variety in mobs, a lot of people refer to being at the 'softcap' for certain zones. It's important to make the distinction between this reference and what I describe in (1) above.
If you're a plate tank, your goal is to stay alive--that means reducing your damage intake. The easiest way to do that is to strap on a shield and attain as much AC and Shielding as you can. If you are a silk-wearing caster, you can view AC like a warrior views mana--no concern whatsoever--and honestly, even as an SK, I never pay attention to mana. It's just something that happens to be on my gear that I can make use of, but is never a criterion for choosing my upgrades. For those in between these extremes, just remember AC benefits plate tanks the most, hands down, and the further away from a plate tank class you are, the less it does for you.
IV. WHAT IT MEANS
The numerical analyses above focused on tanks for a reason: It demonstrated the power of mitigation. Non-tanks do not mitigate to DI3, or DI4. They mitigate hits down with much less frequency, and take a lot more DI20 hits. Because of that, shielding is of very little importance to them, because it is reducing only a small and infrequent portion of their incoming damage. It doesn't save their lives, or increase their survivability, to the same degree that it does that of tanks--and in most cases, it won't affect things at all.
For non-tanks to increase survivability, their best bet is to go for +Avoidance, and +HP. Avoidance makes an attack miss you completely--thus, it's the fastest way to raise durability for classes that can't mitigate. I've never told anyone that Avoidance should be restricted; it shouldn't. Likewise, adding pure HP is important, because if you're constantly being hit for a high DI value, you need to be able to just absorb the hits since you cannot effectively mitigate them to lesser values.
As said above, the primary goal of a tank is to lower the likelihood of his death. High AC does that, and does it pretty well. Shielding also has a dramatic effect on directly lowering incoming DPS. Taking a whole lot of hits for minimum, few maximum hits--particularly in a row--and mitigating those maximum hits down to even lower values via shielding? That's a tank.
What's unfortunate is that people don't seem to understand how much less useful shielding itself is to a class that gets hit for high DI/near max every time. Its value is maximized by people who are taking incoming damage over a long period of time--not a few rounds here or there. There is no argument to be made that can claim that the 15 seconds other classes get hit--and often die--compares at all in relative value to anywhere from 5 to 20 minutes of tanking.
Sony realizes it, and itemizes appropriately. Shielding is much more prevalent on plate and chain gear; avoidance is prevalent on leather and silk gear. Yes, some plate gear has avoidance, and some silk gear has shielding--but silk gear sometimes has Haste, too, and plate gear has mods that are useless to 4 of the 5 classes that can use it.
The greatest limiting factor on progression revolves around the tank/healer dynamic.
The more damage you can absorb, mitigate, and replenish, the greater your chances of surviving, and progressing. The area in which we struggle the most has, traditionally, been that requiring tank depth.
Weaker tanking means more resources necessary to keep them alive, meaning fewer resources to fulfill other roles.
Stronger tanking means less resources necessary to keep them alive, meaning more flexibility, more available content, and the ability to handle more complex events because you can divide and assign your resources more effectively.
This isn't rocket science--look at the trial of Hatred. Compare two people: Hrendra tanking pups in Hatred and needing one cleric in group, or a newcomer tanking pups and requiring 2 clerics in their group. That's not to toot my horn--that's to demonstrate that it's a significant difference in even a basic raid.
A lot of times, we blame healers when a tank dies--there are limits to what they can accomplish. It is NOT always the healer--it's a dynamic. It's interdependency. You could have 20 healers in a raid, and if the tank isn't strong enough, he'll die anyway, especially in important situations like tank switches, when the tank absolutely must be able to survive during the time frame it takes healers to retarget and begin casting.
Not everyone has the time to research as I do, and I understand that. I'm a bit of an obsessive student of the game. However, I'm hoping that this document will enable people to better understand how damage intake works in Everquest, and why tanks--myself included--get up in arms about shielding. It's a wonderful example of how people should use more common sense when bidding/distributing loot, in both raids and xp groups. I encourage everyone to take the steps to learn about the game around them. Your class boards are a wonderful way to start, and Google can provide links to any of them if you have even the most basic search engine skills.
