What is Defense Skill?
Defense is a skill you have that helps you avoid damage from your opponents.
The official definition from Blizzard is:
- Increases the chance of being missed by an attack. - 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 70 (350 defense) against level 60 (300 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 5xLevel (300 at level 60, 350 at level 70, etc.)
The real 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 virtually no critical hits in combat. It's generally spike damage - multiple critical hits and crushing blows - that kill tanks in raids. Minimise the number of critical hits and crushing blows you take, and you help your healers keep you alive with much more ease.
What's that, critical hits and crushing blows? We all know about critical hits - Someone crits you for 200o. You will also see when a creature lands a crushing blow on you - Creature hits you for 2000. (crushing). What's the difference?
Crushing blows
Crushing blows are defined in detail here: Crushing Blows. Any creature 3 levels higher than you will land crushing blows, assuming your defense skill not including defense from gear is at maximum for your level (350 at level 70.) This should not be an issue for a warrior. At level 70, you will be subject to crushing blows from level 73 creatures. Note that anything defined as World Boss is considered 3 levels higher than you for purposes of you hitting it and it hitting you - just with a bazillion HP, special abilities, etc. This is in effect throughout the game, so Onyxia and BWL/MC bosses will still crush you at 70.
Once upon a time, there was the "15% rule". And the 15% rule said this: "There is a minimum 15% chance to take a crushing blow. Period." And so we believed that the Shield Block ability was the silver bullet for the 15% rule. Well, now the protection paladin is here, and they have definitively proved that there is no 15% rule. In fact, all there is to it is that if you are at your level maximum (or more) defense skill against a mob 3 levels higher than you, then that mob has a 15% chance of crushing blow on its combat table. We now know for a fact that if you stack your total dodge+parry+block+chance to be missed to 102.4% you cannot receive a crushing blow. In warrior terms, nothing changes except our understanding of how combat works in WoW. Crushing blows and Shield Block are discussed here. Reading up on the combat table would be a good idea (and it also explains that bit about 102.4%.)
Critical hits
Critical hits from mobs hit you for 200% of the mob's normal damage. Only white damage attacks (regular autoattacks) can be critical hits - spells and special abilities such as mortal strike cannot crit you (though player spells and specials can crit in pvp, etc.) So far as we know, observed, and analysed, most creatures get (5 x level) weapon skill for attacking, and don't get bonuses to critical hit. We also know that if you have defense equal to your opponent's weapon skill, then your opponent has a 5% chance to critical hit you. So, for a level 70 player with 350 defense against a level 70 mob (level*5 = 350 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 the majority of level 70 creatures only have 350 weapon skill, level 71 has 355, and so on, but there are some exceptions. This matches observed behaviour to date.
Looking at this table, we can see that with 475 defense against a level 70 opponent that has 350 weapon skill, you should reduce the chance that you'll take a critical hit from them to zero. Similarly, we see that you should reduce the critical hit chance to zero against level 71 opponents at 480 defense, 72 at 485, and 73 at 490. Anecdotally, there have always been reports of critical hits landing "once in a blue moon", regardless of how much defense you have. Blue has said that
Which of those is it? We just don't know. In any case, the phrase "crit immune" is an extremely bad choice; don't use it. We can look at the Malicious Instructor in Shadow Labyrinth that is known to land crits on 525 defense (7% crit reduction in theory.) Quite possible that this mob has an increased basic crit chance since it isn't even level 73 and consistently crits 490+ defense. Also possible that their "Mark of Malice" debuff either increases their crit chance, or reduces your crit reduction - though there is no visible change in your defense skill while you have the debuff (like the Twin Emperors' Unbalancing Strike does.) Either way, this mob consistently crits "uncrittable" defense skill warriors, but we can only speculate whether the Instructor has more than a 7% crit chance so that it beats 525 defense (soft cap), or maybe 5.75% to just beat a hard cap that you reached at 490.
So, once in a blue moon, or just higher crit chance? This is the realm of opinion and holy war amongst warriors. Personally, my money is on a higher crit chance given the far greater number of mobs that do not land crits on 490 and higher defense, rather than an exception case in the combat mechanics that makes little sense from either a logical or programming perspective. I also have over 40,000 attacks from world bosses in CombatMonitor, from Karazhan to Tempest Keep, without even one critical hit (but there are crits from Malicious Instructors.) Statistically speaking, it's still possible that I am just really lucky. But not very probable. This is, of course, my opinion and not a fact.
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 70, you get X/2.4 defense skill for that (e.g. 24 defense rating => 10 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?
Good question.
So, how much defense? This "magic number" everyone talks about. Given that your defense from gear does not affect the chance that you will take crushing blows, we focus on critical hits, which we can effectively eliminate. If you want to absolutely minimise your chance of being critically hit, the number is 490 defense skill - that will reduce the critical chance of any mob, boss or not, to zero unless they debuff defense or have a higher crit rate. Pre-Burning Crusade, warriors tanked world bosses every day with anywhere between 300 and 430 defense. These days, a raid boss critting you is probably going to cause an unrecoverable drop in your health and wipe the raid.
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:
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:
def = 25 x ChanceToCrit
Where ChanceToCrit is the player's listed chance to critical hit. So, at level 70, facing an opponent with a 25% chance to crit, you would need 625 defense from gear to counter their chance to critical hit. Keep in mind that, if you are level 68 fighting level 70, you need two more levels worth of defense, or 10 points, to make up for the level difference. Or, if you are level 70 fighting level 68, your chance to critically hit is actually 10 x 0.04 = 0.4% higher than your listed chance to critically hit.
And so, 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!