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.