[General] The effects of expertise on mitigation

As before, here's the summary: Expertise is the single best stat to stack to reduce the chance of spike damage.

Expertise is understood by most warriors to be their best threat stat. Druids get a lot of good use out of it too. Paladins...well, not as much, threat wise. Since only about 35% of a prot paladin's threat is generated from melee attacks (melee + SoR is basically it) and it doesn't affect taunting, it's not quite as useful as it is to druids or warriors. For druids, all of their attacks are affected by expertise, and expertise is twice as effective as hit rating for threat generation until you reach 24 expertise skill (or 96 expertise rating). It's one of the best if not the best stat to stack for threat.

For Paladins though, it is still pretty useful, and on a per-point basis expertise is almost as good as spelldamage for threat. But that's not the important thing.

For tanks the biggest issue is not how much damage they take per hit; tanking gear tends to normalize to the point where all types of tanks tend to take close to the same damage given the same boss and assuming the same or similar gear levels. What kills a tank is burst damage. In some fights that burst damage can be avoided or otherwise anticipated so that no other damage is taken at the same time; an example would be Magtheridon, where he brings the ceiling down. The raid can stop DPS until after a blast nova, then get him to 30%, cause a lot of raid damage and then resume as normal. In other situations certain bosses will be reduced more by certain classes; Morogrim and Gruul do mostly physical damage, so a druid's high armor comes into play more as a reduction (as an example). Or Prince and a Paladin, where holy shield all but guarantees every attack will be blocked and reduced more. That's one way to reduce spiky damage - make sure that the damage you take is as low as possible.

The other is to reduce the chance of having that spiky damage occur at all. This means we need to understand where spikes come from. One big source is from being parried by a mob. When a parry occurs the mob that parried partially resets their swing timer so that the next attack comes in faster. How much the swing timer is reset or reduced depends on when the parry occurred relative to the swing timer, but in general it's a 40% reduction to the next attack's speed. When this happens, especially in succession, you'll start seeing a lot more spikes of damage - and when this is combined with a boss's special attacks it can quickly lead to too much damage for a healer to overcome. For many fights you can get a parried attack which can lead to two normal melee hits + a special in the space of 2 seconds, and depending on the boss that can be quite a large amount of damage, somewhere on the order of 15-18k. And bosses on average have a 15% parry chance, so this can happen with some frequency. These attacks do enough damage that it's not enough to block them - avoiding them in some way is the only way to reduce that spike. For our purposes, we'll define a spike as being hit by a normal attack, being parried, and then being hit by a normal attack again.

Every 1 point of expertise skill (or 4 points of expertise rating) reduces the chance that you will be parried by .25%. So it takes 4 points of skill or 16 expertise rating to reduce parry chances by 1%. It also takes 19 points of dodge rating to give 1% dodge, which is the best avoidance stat you can stack. So, what are the chances that you will be hit by a normal attack, get parried, and then get hit with another normal attack? The normal attack chance is simply (1-avoidance). The parry chance starts at 15%. Therefore, the chance that you will be parry gibbed is:

(1-avoidance)*(parry chance)*(1-avoidance).

The typical avoidance level for T4-T5 bears and paladins is about 40%. With no expertise at all, you have a 5.4% chance of a parry gib occurring. With each 1% avoidance you reduce that chance by .18%. With each 1% of expertise, however, you reduce that chance by .36%. Thus, expertise is nearly double as valuable as avoidance in stopping a spike from occurring. This pattern holds true for all levels of expertise vs. avoidance:



Admittedly, another great way to deal with spike damage or the chance that you'll take it is to stack health and mitigation so that the spike damage can be more easily dealt with. That is the concept that effective health is built around, and it's always better to maximize your effective health instead of sacrificing it for the purposes of something like threat. That all being said - expertise is the best option for reducing the chances of some types of burst damage killing a tank.