Combat rating system

Combat rating is a mechanic introduced in patch 2.0.1 which are for combat stats on items that were previously percentage based. Rating is converted linearly to percentage; the number of rating points needed to gain one percentage point increases with level and varies depending on the stat, but is the same for all classes. Ratings were introduced because percentage based itemization would have scaled excessively when The Burning Crusade was released and the level cap rose to 70.

Combat rating types
The following combat ratings are currently in use:

Unlike fixed percentages such as 2% critical strike chance, combat ratings diminish in potency as characters increase in level. 2% crit is the same at every level, while 28 critical strike rating grants 4% crit at level 34, 2% crit at level 60, and 1.27% crit at level 70. This allows Blizzard the ability to create and add new and better items to the world without eventually reaching a point where every character has a 100% chance to critically strike.

All classes convert ratings into attributes at the same ratio. Below are the conversions for combat ratings at each expansion's level cap:

Notes:

Diminishing returns
Avoidance stats suffer from diminishing returns ("DR"), meaning the rate of return for each point contributed from gear is not constant. The in-game tooltips will show the undiminished value, but the amount actually received is lower. Base stats and talents are not affected by DR, but stats from gear are diminished.

The formula to calculate the diminished part of a stat is as follows: 1   1   k — = — + — x'   c   x where:
 * x' is the avoidance after DR.
 * x is the avoidance before DR.
 * c is a class-specific cap of the stat.
 * k is also a class-specific constant.

The table containing the class-specific caps for parry (Cp) and dodge (Cd) is as follows: