What's with all these Crazy Numbers? A look at Combat Stats, and the math behind them, in the World of Warcraft 
This is likely the question many of you will be asking yourselves when you first load up World of Warcraft and look at some of your equipment. I know I did. When you first explore your character sheet there is a lot of information there that is not really explained in many places. No matter if you are a new player checking out the World of Warcraft for the first time, or a returning player that just needs an update on what has changed in Mists of Pandaria.
The obvious changes from the original method are the really old original +X% to crit and +X% to hit are gone. Instead you will find standardized numbers like +14 to critical strike and +22 to hit. These numbers then convert from base numbers to percentages using a conversion factor based on your level and a conversion factor. Don't worry, all will be explained.
If you already know what all the stats do and just want to know how much of each you need to get a 1% increase jump to our Combat Ratings in Mists of Pandaria chart to show what you need for each stat at level 90.
What are the stats and what do they mean?
Lets start with a short explanation of what each of the major combat rating stats are and what they do.
Important Note: When referring to caps or numbers needed against enemies these are all based on you being level 90 and the opponent being a boss (meaning they count as 3 levels higher).
Critical Strike  This stat boosts your chance to get a critical strike with each attack. At level 90 you require 600 crit rating to earn 1% critical strike chance.
Haste  Decreases your attack speed or casting speed by a percentage. With 10% haste a 2 second attack or cast would take 1.8182 seconds. Speeds are rounded to 4 decimal places. The calculation is:
Hastened Speed = Original Speed / [ 1 + (Haste Rating % / 100)]
Also different Haste effects stack (all haste ratings count as one effect), so a speed boost from berserk or any other effect provide a stacking boost, calculated by this equation:
Hastened Speed = Original Speed / {[ 1 + (Haste Rating % / 100)] x [ 1 + (Haste Rating % / 100)]}
Parry  Parry is the ability to turn aside an incoming melee attack that is an ability that most melee classes can learn. It is however primarily a tanking stat as it is meant to reduce the incoming damage that they suffer.
Dodge  This is a percentage chance to completely avoid an incoming attack. As with Parry this is primary a Tank stat.
PVP Resilience  This stat was originally introduced in the Burning Crusade expansion and provided both PVP defense and PVP damage. As of Mists of Pandaria it now only provides a damage reduction. Basically you will require 310 PVP Resilience to lower the damage you suffer in PVP by 1%. That is the simple version, it is slightly more complicated than that, but you can use that as a basic understanding. If you want to know more check out the Diminishing Returns section below.
PVP Power  This stat is the attack portion from the original Resilience split out to be its own stat. It provides a boost to the damage you cause to PVP enemies. It requires 265 points to cause 1% more damage.
Expertise and Hit
Expertise and Hit will be dealt with together since they work on the same stat to percentage conversion as of Mists of Pandaria. At level 90 you will require 340 expertise of hit rating to earn 1% of either stat.
Expertise  Expertise lowers the percentage chance that your melee or ranged physical attacks will be dodged or parried. This means it is important for all melee DPS or Tank classes as well as for Hunters. This is an important change in Mists of Pandaria as previously Hunters did not require expertise.
Expertise has two key components and therefore two key numbers to aim for. At 7.5% Expertise enemies will lose the ability to dodge your attacks, this is generally referred to as the expertise soft cap. At 15% Expertise opponents lose the ability to parry your attacks, this is generally referred to as the expertise cap or the expertise hard cap. For most classes and specs only the softcap is important, as when you attack an enemy from behind they can not parry anyway. The only spec that should even consider the hard cap is tank specs, however even then generally the soft cap is enough.
Hit  This stat raises your percentage chance to hit an opponent with your attacks. There are two sets of hit numbers to worry about depending if you are a melee class or a caster class and if you are concerned about PVE content or PVP content. For PVE content if you are a melee class you require 7.5% hit to never miss an attack or 15% hit if you are a caster class. For PVP content you require 3% hit as melee or 6% as a caster.
Required Expertise and Hit at Level 90
How the hit table is determined in World of Warcraft is based off of a 3% miss chance if the opponent is equal level to you, hence the different cap for PVP since players will be the same level as you are. Then for each level that the opponent is higher than you, it gets progressively harder to hit them. What this means in terms of the game at level 90 is that you have the following miss percentages, which also equate to the expertise required to not allow them to dodge, and therefore need the amount of hit or expertise as shown. In general players only really worry about hitting the boss cap which is 3 levels above you at level 90, therefore that is the number you should aim for.
Opponents
Level 
Melee Hit Cap / Expertise SoftCap

Required Hit / Expertise to achieve %

Caster Hit Cap / Expertise HardCap

Required Hit / Expertise to achieve %

=

3%

1020

6%

2040

+1

4.5%

1530

9%

3060

+2

6%

2040

12%

4080

+3

7.5%

2550

15%

5100

What are Combat Ratings?
What you will notice on your gear is that you will see items with large numbers on them, such as +450 Hit or +400 Crit. Obviously these numbers need to convert to the percentages that show what they do for you, otherwise players would not be able to know at a glance where they are. For example knowing that you need 7.5% hit for raiding is easier than knowing you need 2550 hit rating. All of these numbers are collectively called the Combat Rating System by Blizzard.
Therefore you should think of Combat ratings as a conversion system for the numbers you see on gear to a percentage that is more meaningful for you.
Historical Information
Under the old original system way back in vanilla WoW almost everything had a set percentage (%) increase, this lead to problems as you increased in level. It also made scaling gear very difficult as what was better, a level 50 item with +2% to crit or a level 60 item with +2% to crit. Neither, they were the same. Also there was no place to go but make the numbers even higher.
To resolve this, the percentage was changed to a number value (as discussed above), and then the number value correlates to a calculation to figure out the amount it will increase your stat. By using this method an item can be given a value which is appropriate to the level, however it will scale down as you level. This means that with a +14 to critical strike chance at level 60 gives you a 1% increase it takes +22 at level 70 to get the same 1% increase.
This allows better items at any level in the game to be added or balanced without creating situations that would grant players obscene +x% bonuses at high levels with low level gear. It also allows the creation of really good low level gear, that will be great at level but scale down in effectiveness forcing its replacement.
Combat Ratings at Level 90 in Mists of Pandaria
Combat Skill Ratings


Rating

Effect of Rating

Points
required at 90 

Expertise

1% chance for enemies to Parry or Dodge your attacks.

340


To Hit

+1% chance to hit

340


Critical Strike

+1% chance to critically strike target

600


Haste

+1% haste with melee or ranged attacks, or casting

425

Defense Skill Ratings


Rating

Effect of Rating

Points
required at 60 

Dodge

+1% chance to dodge

885


Parry

+1% chance to parry

885


PVP Resilience

Take 1% less damage taken in PVP 
310 

PVP Power  +1% damage caused in PVP  265 
Diminishing Returns
Several stats suffer from something called diminishing returns. What this means is that as you get more rating in one of those stats the rating does progressively less for you. A very simple example to consider that explains the basics would be a stat that requires 100 points to get a 1% boost, but 210 to get a 2% boost, then 330 to get a 3% boost. The stats that suffer from diminishing returns in World of Warcraft use much more complicated math than a 10% increase per percentage required, but you should understand the basics from the example above.
Dodge / Parry  Both of these stats are on a diminishing return system that becomes steeper the more of it that you have, however they are both on different scales. Dodge is hit much harder and sooner by diminishing returns that Parry so your priority will always be Parry over Dodge to a certain ratio. The math for this is extremely complicated and the best write up I have seen on it is over at Sacred Duty, which you can check here: Diminishing Returns in MoP.
Essentially though, you will want to keep your parry rating at about 2 to 2.5 times what your dodge rating is. Just a few examples of where you want stats to be so that they are suffering roughly equally from diminishing returns are: 10% Dodge / 20% Parry, 15% Dodge / 40% Parry, 20% Dodge / 60% Parry. As you can see as ratings get higher the ratio changes.
PVP Resilience  As mentioned in the initial PVP Resilience section, calculating the real percentage of damage reduction provided is not as simple as 310 points per percentage. For those that really want to get exact values, how it works is you need 310 to take 1% less damage as compared to the last full percentage less of damage that you took. This means that 310 points grants you 1% less damage initially from 100% to 99%, but the next 320 points grants you 1% less damage than the 99% not of the 100% initial.
Looking at bigger numbers than 1% at a time makes it easier. While 310 points initially takes you down to 99% damage suffered, it takes a bit under 3300 points to only suffer 90% damage. As you can see the difference is not that significant as at 310 per 1% it should only be 3100 points. While the percentage difference is small but is important to note since the more PVP Resilience you have the less each increase to it makes a difference.
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