This article is copied straight from TankSpot
http://www.tankspot.com/forums/f63/41796-defense.html by Satrina.
What is Defense Skill?
Defense is a skill you have that helps you avoid damage from your opponents.
The official definition from Blizzard is:
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-Increases the chance to dodge, parry, and block.
-Decreases the chance of being affected by a critical hit.
-Decreases the chance of being affected by a crushing blow.
In practice, we can see by equipping an item with +defense that one point of defense skill increases your chance to dodge by 0.04%, chance to block by 0.04%, and chance to parry by 0.04%. It has been stated by Blizzard that a point of defense reduces your chance to be critically hit by 0.04% - though your chance to be critically hit is relative to your opponent's weapon skill (and any +crit items they are wearing, of course). It's been tested to determine that a point of defense increases your chance to be missed by 0.04%. Note that if you are higher or lower level than what you are fighting, the difference counts, too. A level 80 (400 defense) against level 70 (350 weapon skill) gains the benefit of the extra 50 defense versus that opponent automatically (+2% miss, dodge, parry, block; -2% chance to take a crit).
In various places, I'll use a term called level maximum defense. This is simply your maximum defense skill for your level, or (5 x level) - 350 at level 70, 400 at level 80, etc.
One primary benefit to defense skill is the reduction in critical hits. You can't dodge or parry every attack, but you can make it so you take no critical hits in combat. Minimise the number of critical hits and crushing blows you take, and you help your healers keep you alive with much more ease. The other benefit to defense is its increase to your avoidance skills (chance to be missed, chance to dodge, chance to parry), and its increase to your chance to block. Block is not avoidance; it is a mitigation
Crushing Blows
Not much of a concern in Wrath of the Lich King, except maybe for tanks who are still levelling up. Read here: Crushing Blows.
Critical hits
Critical hits from mobs hit you for 200% of the mob's normal damage. For PvE creatures, only white damage attacks (regular autoattacks) can be critical hits - spells and special abilities such as mortal strike cannot crit you. Player spells and specials can crit in PVP, of course. So far as we know, observed, and analysed, creatures get (5 x level) weapon skill for attacking. We also know that if you have defense equal to your opponent's weapon skill, then your opponent has a basic 5% chance to critically hit you. So, for a level 80 player with 400 defense against a level 80 mob (5 x level = 400 weapon skill), the mob has a 5% chance to critically hit the player. This is, of course, subject to exception for critical hit enhanced monsters, or monsters that debuff your defense. We will for now work on the assumption that level 80 creatures only have 400 weapon skill, level 81 have 405, and so on, but there are some exceptions. This matches observed behaviour to date.
Looking at this table, we can see that with 525 defense against a level 80 opponent that has 400 weapon skill, you should reduce the chance that you'll take a critical hit from them to zero. Similarly, you should reduce the critical hit chance to zero against level 81 opponents at 530 defense, 82 at 535, and 83 at 540. We call this level of defense the critical minimum.
In the above paragraph, we say "should" to all of this. Of course, we always have the reports of some mob that lands a critical hit on a tank that is well past the critical minimum for the mob that was attacking. We can look at the Malicious Instructor in Shadow Labyrinth in Outlands that was known to land crits on level 70 tanks with 525 and higher defense skill. It's notable that all of these mobs that manage to crit people that are at or past the critical minimum are rogue types. From this we infer that at least some rogue type mobs have an increased chance to critically hit. That doesn't answer the question as to why crits are landing on tanks with extremely high defense scores. We find a probable answer in this quote from Kalgan speaking specifically to critical hit reduction from defense:
Source: Kalgan
At 490 defense, the math caps out. Defense won't mitigate beyond that.
(Note that the quote comes from the Burning Crusade era, where the maximum was level 70 and so the critical minimum for level 73 mobs was 490). Based on this, it's quite possible that the critical hit reduction mechanic bottoms out at 5.6%, and so a mob with more than 5.6% chance to critically hit will have a chance to critically hit any tank, regardless of defense skill. The chance would be small (chance to crit - 5.6%), but not zero. We don't know that for sure, though. Since we do occasionally see mobs landing critical hits on tanks with extremely high defense, there's a good probability that is the reason, however.
Diminishing Returns
Defense increases your chance to be missed by an attacker, dodge attacks, parry attacks, and block attacks. Of these defenses, three of them (miss, dodge, and parry) are subject to diminishing returns. That is, every point of defense you add gives you slightly less chance to miss, dodge, and parry than the previous point of defense gave you. Diminishing returns on avoidance is discussed in depth here: Diminishing Returns - Avoidance.
Defense Rating and Defense Skill
One thing to not mix up is defense rating and defense skill - they are related, but different. When you look at an item, it will say "increases defense rating by X". The amount of defense skill you get from an amount of defense rating changes with your level. If you're level 80, you get X/4.92 defense skill for that (e.g. 24 defense rating => 4.88 defense skill). Items list how much rating the item gives. Your skills page and character panel show how much defense skill you have.
How much defense do I need, then?
540 if you are raiding. There's no reason why you would not be uncrittable as a raid tank, ever. Beyond that, it's probably a good idea to be uncrittable for against what you're tanking to be nice to your healer, but it probably won't be a huge problem unless you really undergear the content.
Once in a Blue Moon
I almost left this out of this iteration of the guide. It's still debated in some quarters, though, so I'll keep it around for now. As noted above, certain mobs land ctitical hits on tanks that are beyond the critical minimum for the mob's level. This has led to a belief for some that there's always a chance of a critical hit landing "once in a blue moon", regardless of how much defense you have. People will quote numbers like 0.01% or 0.000001%.
- - If the chance were 0.01%, or even 0.001%, we'd see a lot more reports of critical hits on tanks across all servers, given the size of the playerbase
- - A lot more players would be reporting being one-shot by raid bosses
- - Reports of mobs landing critical hits on tanks beyond critical minimum always are the same ones, and they always seem to be the same type (dual wielding, rogue-type) of mob
Other ways to get critically hit
No matter which theory on crits vs. defense you believe, the crit you just took may not be related to that at all.
You sat down. Probably the leading cause. You miskeyed, sat down, and took a crit. Best part, the exchange happened in under a second, so your computer didn't even have time to render the sit animation, in all likelihood. Oops. Unbind sit from a hotkey, or at the very least, bind it to something inconvenient to hit.
Mind control. Feral Druid. Leader of the Pack. Mind Control. Boom.
Some debuff. Twin Emperors in AQ40 were the first place we saw this. Debuff the tank's defense by 100, and the crits begin. It's happened before, it'll happen again.
Defense skill and PvP
Some people say that defense works differently in PvP than it does from PvE, and some say it doesn't. Since it's been stated by Blizzard that it works the same in both, I'll go with that I think. The problem is that since players can and do collect gear to increase their critical hit chance, chance to hit, weapon skill and so on, that you just need that much more defense to counter their critical hit chance. The question becomes, how much do you need, and is it worth it to get there?
Let's start with what we know:
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1 defense = -0.04% chance to critical hit (verified by Blizzard)
From that we can see that 25 defense counters 1% chance to critical hit. With that in mind, the amount of defense you need from gear to counter another player's chance to critical hit would be simply:
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def = 25 x ChanceToCrit
Where ChanceToCrit is the player's listed chance to critical hit. So, at level 80, facing an opponent with a 25% chance to crit, you would need 625 defense skill (3075 defense rating!) from gear to counter their chance to critical hit. Keep in mind that, if you are level 78 fighting level 80, you need two more levels worth of defense, or 10 skill (50 rating), to make up for the level difference. Or, if you are level 80 fighting level 78, your chance to critically hit is actually 10 x 0.04 = 0.4% higher than your listed chance to critically hit. It's pretty unlikely that you'll get enough defense to mitigate critical hits from other players. On the other hand, all your defense will reduce the chance that they will critically hit you, of course! Point for point, resilience is better than defense for reducing critical hit chance, which is why you find it on all the PVP-oriented gear.
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