There is a LOT of talk about survival, of course, it's a tanking forum! There are a great many great minds here, and still more followers who are not interested in doing out the crazy math themselves, they're looking for people to boil it down and tell them what they *need* to know. I want to present the information concisely, but in detail, those who are already fluent in these details, bear with me in the early sections.
Preface: Tanking is an act of balancing need. You need to balance survival with threat, while meeting the needs of both. Too little threat and you could be unkillable, it won't matter. Too little survivability and you can have all the threat in the world and you'll just get steamrolled by anything you pick up.
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Within survival there are several places we get values from. I want to start from as simple and general as I can manage, and progress to a more specific breakdown of stats, hopefully with a mind for being plainly accessible to anyone, regardless of aptitude with math, logic, or probability.
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What we're up against:
In the simplest terms we're taking damage of one of two types, with some degree of frequency: Physical damage and Magic damage. We can also split the form of delivery into direct damage, or damage over time (physical DoT's are usually called bleeds).
There are two ways we handle incoming damage: we avoid it or we reduce (mitigate) it. So, there are 3 general pools that we manage to do this: health, mitigation, avoidance.
Health
This is, plainly, your lifespan. It will be drained when you take damage, and filled when you are healed. When it hits zero, you are dead. A dead tank holds no threat and saves no lives. This is a simple number scale value, and we only affect it's maximum limit. The two ways we do this is with stamina from gear and buffs, and multipliers from talents and effects that will buff our stamina or our health directly. (Note: this is not the same as effective health, that is a more complex concept, I'll discuss later).
Mitigation
Anything that reduces the damage taken falls into this category. For physical damage, we rely on Armor primarily. We also get stances, buffs, and talents that will reduce all or specific types of damage by a flat percentage. Resistance functions as a sort of armor against the specified school of magic, though it also affects a chance for large chunk percentage-based reduction of that kind of magic. Shield Block is a mechanism that belongs in this category, it provides a chance to reduce incoming physical hits by a set amount based on your gear.
Avoidance
Avoidance includes anything that allows you to not take damage in its entirety. Monsters can miss you when they attack with magic or physical strikes, you can also dodge incoming strikes, or parry them. These are the major forms of avoidance. Generally, there are no forms of dodge or parry against magic damage alone, though some magic damage sources are linked to avoidable attacks. This avoidance is achieved through defense rating/skill, dodge and parry ratings, and agility.
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Most above this point is fairly common knowledge, but application is rather more difficult. In reality, our survival is compound of all of these elements working together, each in their part. So I want to discuss where each element fits. The following section will build composites towards a more encompassing picture.
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Effective Health / Guaranteed Survival
The concept is very simple, in the absence of all random possibility, bad luck, random generated outcomes, and anything else that could happen, it is valuable to know what will be unaffected in your survival tools. This value can be a combination of elements, but the most simple an inalienable is the combination of static mitigation (armor, resistance, % damage reduction) and health.
As a simple illustration:
You are tanking Boss X. He hits for 50,000 damage every 2 seconds. You have enough armor to give you 60% damage reduction, your stance and talents give you 10% damage reduction to all sources, and you have 35,000 health. No matter of where you stand, how you position, whether you are stunned, knocked down, afk, or offline, these values will remain in effect.
After armor (50,000 becomes 20,000) and stance/talents the boss will hit you for 18,000 damage every 2 seconds. You will survive one hit. If you do not receive healing, or access non-static protections (read: everything else listed above), you will die to the next hit.
In terms of Effective Health, you could say you have 97,222 health to face against the boss's unmitigated damage.
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The size of the hits being taken will create a jagged steppe-plateau value gained from adding health. In the above example, if you added 2,000 more health, you would survive that second hit, just barely. But if you added 2,000 more health after that, it would make no difference.
<coming eventually, table of approximate ideal EH values sorted by content level> Functionality of Avoidance
Avoidance provides a chance for you to avoid incoming damage completely. Unlike armor or static damage reduction this has three conditions that can limit its function:
1.)
Positioning = a tank cannot dodge, parry, or block (though not avoidance it is worth noting here) any attacks that come from behind. Ideally you will position everything you are tanking in front of you, but there are times where things will end up behind you, even for a moment.
2.)
Status Changes = when you are stunned, incapacitated, slept, knocked down, or when you are casting a spell, you cannot dodge, parry, or block.
3.)
Random Chance = At best, you have only a chance to avoid incoming hits. This is represented in the percentage values on your character sheet.
Note: while the first two points illustrate where you cannot dodge or parry, you
can still be missed, as that is a function of the target's ability to hit you, and does not require your involvement.
So, the practical function of avoidance is to occasionally interrupt the stream of damage that you take to allow your healers some time to catch up or pause in their own casts for efficiency sake. In order to gauge the value of avoidance, it is worth applying simple probability to find out how it translates into the chance to be hit more than once in a row.
The formula for this is: Chance to be hit X times in a row = (Chance to be hit once)^X
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For a quick general check, find the closest value to your total avoidance value (dodge+parry+miss) on the table below:
Code:
(in a row)
Avoidance 1 hit 2 hits 3 hits 4 hits
20.00% 80.00% 64.00% 51.20% 40.96%
25.00% 75.00% 56.25% 42.19% 31.64%
30.00% 70.00% 49.00% 34.30% 24.01%
35.00% 65.00% 42.25% 27.46% 17.85%
40.00% 60.00% 36.00% 21.60% 12.96%
45.00% 55.00% 30.25% 16.64% 9.15%
50.00% 50.00% 25.00% 12.50% 6.25%
55.00% 45.00% 20.25% 9.11% 4.10%
60.00% 40.00% 16.00% 6.40% 2.56%
65.00% 35.00% 12.25% 4.29% 1.50%
70.00% 30.00% 9.00% 2.70% 0.81%
75.00% 25.00% 6.25% 1.56% 0.39%
80.00% 20.00% 4.00% 0.80% 0.16%
85.00% 15.00% 2.25% 0.34% 0.05%
90.00% 10.00% 1.00% 0.10% 0.01%
95.00% 5.00% 0.25% 0.01% 0.00%
Your dodge and parry chances will be approximately what they appear as on your character sheet (-0.2% per level the target is above you, bosses count as level 83, so -0.6% off each). The easiest way to find your miss chance is to use an addon such as TankPoints, however for a tank at the defense cap it will be approximately 9% after diminishing returns.
I will not dig into the deep scary math of diminishing returns to translate ratings into avoidance percentages, but there are 2 simple points to understand how this affects your survival gained from gear:
1.) Health has a linear value for your survival, meaning you simply add or subtract directly. Bigger damage means you die faster, simple as that. Armor and Avoidance deal with a percentage reduction. As that percentage approaches 100% (complete reduction to 0) each percentage increase will become more powerful in terms of direct survival. I'll use a big jump for simple illustration.
If a boss swing, before mitigation, is 50,000 damage, and your armor reduces that by 40%, you will take 30,000 damage per swing. If you up your armor to reduce that damage by 50%, you now take 25,000 damage each swing or a 17% increase in the reduction. If you then increase it again to now by 60% reduction, each swing now does only 20,000 damage, so from 50% this is now a 20% improvement in damage reduction.
Un-condensed:
Code: Mitigation % Boss Hit Damage Reduction Change
40% 30,000
50% 25,000 -17%
60% 20,000 -20%
Because of this, both armor and avoidance are designed to make that increase in damage reduction scale linearly with rating, rather than making the percentage scale linearly. Contrary to some small circle's beliefs, diminishing returns does not make avoidance somehow weaker than pure health, it is the only thing that keeps it on par.
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2.) Two features determine the relative gain from each avoidance rating: the base exchange rate (X parry rating = Y% parry) and the cap to which the stat diminishes (you cannot have more than 88% dodge, in fact you can never reach that value). Dodge's cap for tanks is ~88% (slightly different for Druids) while Parry's cap is ~47%. As of patch 3.2 the base exchange rate for the two is the same, so it is only the lower cap that keeps Parry rating from giving the same value as Dodge rating.
When in doubt, just try on the gear and see what happens. Alternately, addons like TankPoints provide tools to try plugging in what would happen if you added, say, 16 more dodge rating or 16 parry rating.
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The Big Picture
In order to see how all the elements come together, I will first describe in simple points, then try to illustrate in a few different examples, how the three elements come together.
Health = is what creates your potential for soaking damage
Mitigation = is what reduces the severity of hits to make your health go farther. (armor, passive reduction %, resistance, block)
Avoidance = is what reduces the frequency of damage taken, creating gaps for healers to catch up. (dodge, parry, miss)
If you are a visual sort of person, imagine incoming damage on a chart. Each successful hit that lands is a peak. Avoidance makes the peaks fewer and more frequently spaced out. Mitigation makes the peaks shorter uniformly, and block will make some of the peaks shorter. In terms of survival relative to your health, it will take larger peaks, closer together, to kill you, and the more health you have the larger/more peaks you can survive. See the diagrams below:
When the total of the peaks in a certain time period surpass your health plus the heals in the same time period, you die. Mitigation reduces each peak, block reduces some of the peaks by more still, and avoidance reduces the amount of peaks grouped together that you typically see.
mind if I link to it from my gear guide?
