Effective Resistance and Average Mitigation Edit
The effective resistance rating of the target was calculated as follows:[citation needed]
effective resistance rating = Rb + max((Lt - Lc) * 5, 0) - min(P, Rb)
Rb - target base resistance (as advertised on the character sheet)
Lt - target level
Lc - caster level
P - caster spell penetration
In PvE boss fights, the player level was usually equal to or less than the boss level, and no spell penetration was in effect. Therefore, in this situation, it was safe to assume that the player's effective resistance was the same as the base resistance (from the character sheet).
Note that this formula rendered level 83 bosses as having 15 magical resistance against level 80 player attacks. Unlike all other magical resistance in the game, this resistance from level difference could not be overcome by spell penetration.
The percentage of magical damage that was mitigated on average was calculated by the following equation (note that this has changed in Wrath-Logo-Small Wrath of the Lich King):[citation needed]
Damage reduction percentage = 100% * effective resistance rating / ( K + effective resistance rating )
For level 80 mobs this constant K was 400. For level 83 boss mobs K was believed to be 510.[3]
Binary Resist and Non-Binary Resist Edit
Spell resistance was the second of two rolls to determine whether or not a spell was resisted. (Spell hit was the first roll.) The spell resistance roll would calculate in one of two different ways: one way for Binary Spells, another way for non-Binary spells. Non-binary spells were spells which only dealt damage (like [Fireball] or [Shadow Bolt]). Binary spells were spells which dealt a debuff (like [Frost Nova] or [Vampiric Embrace]) or, for some other reason, needed to take full effect or no effect at all.
For binary spells Edit
Resistance reduced the chance for the binary spell to land by a certain percentage. Spell hit would not reduce this chance. It was assumed that this percentage was exactly the damage reduction percentage given above.[citation needed]
For non-binary spells Edit
Resistance increased the chance to suffer only a fraction of the spell's normal damage. As of Wrath of the Lich King, the possible percentages of damage resisted were weighted in sections of 0%, 10%, 20%, ..., 100% averaging to the same overall damage mitigation.[3] However only the closest three or four sections would ever occur at a given resistance value. This allowed a sufficiently high resistance to guarantee a certain resist percentage, even in the worst case. DoT spells were considered non-binary and the partial resistance calculation was applied on each tick.
The probability of an x% resistance was given by the following formula (AR is the average resistance calculated above):[citation needed]
P(x) = 50% - 250%*|x - AR|
If the probability was less than 0, it could not occur.
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