Binary Resist and Non-Binary Resist
Spell resistance is the second of two rolls to determine whether or not a spell is resisted. (Spell hit is the first roll.) The spell resistance roll will calculate in one of two different ways: one way for Binary Spells, another way for non-Binary spells. Non-binary spells are spells which only deal damage (like Fireball or Shadow Bolt). Binary spells are spells which deal a debuff (like Frost Nova or Vampiric Embrace) or, for some other reason, must take full effect or no effect at all.
For binary spells
Resistance reduces the chance for the binary spell to land by a certain percentage. Spell hit will not reduce this chance. It is assumed that this percentage is exactly the damage reduction percentage given above.
For non-binary spells
Resistance increases the chance to suffer only a fraction of the spell's normal damage. In The Burning Crusade, the percent resisted had a chance to be 0%, 25%, 50%, 75%, or 100%.[1] The probabilities were weighted such that the average damage mitigated worked out to the same percentage as with binary spells. In Wrath of the Lich King, however, the possible percentages of damage resisted are weighted in sections of 0%, 10%, 20%, ..., 100% averaging to the same overall damage mitigation. However only the closest three or four sections will ever occur at a given resistance value. This allows a sufficiently high resistance to guarantee a certain resist percentage, even in the worst case. DoT spells are now considered non-binary and the partial resistance calculation is applied on each tick.
