One of the most interesting changes coming in Cataclysm is the new Parry mechanic. Currently, a Parry prevents 100% of the damage on the current swing and speeds up your next swing. In Cataclysm, a Parry will prevent 50% damage on the current swing, and 50% of the damage on the next swing, and will not speed your weapon.
What's amusing about this change is that it changes the way you have to analyze tanking. Boss swings are no longer independent. The damage you take on the current swing depends on if the previous swing was a parry or not. Incoming damage becomes a Markov chain, and you need to play around with matrices instead of simple probability.
There are two possible interpretations of how the second swing after a Parry works, and Blizzard has not seen fit to enlighten us yet. The first interpretation is that the second swing always hits for 50% damage, and cannot be dodged or blocked or re-parried. The other interpretation is that you can still miss/dodge/block/parry the second swing, but damage on a parry or block is reduced even further.
In this post, I'll look at the first Parry version. Here is a graph that shows how damage is reduced as you increase Dodge, Parry, or Block (assumes starting values of 5% Miss, 5% Dodge, 5% Parry, 5% Block):
As you can see, Dodge is the best stat of the three. One percentage point of Dodge reduces damage by significantly more than Parry or Block. Intuitively, this makes sense. After all, Dodge completely negates the attack, while you still take 70% damage on a Block. If you had 100% Dodge, you'd take 0% damage. 100% Block would be 70% damage. As for Parry, if you had 100% Parry, you would be taking 50% damage all the time.
However, in this scenario Parry behaves slightly differently than Dodge or Block. Dodge and Block are linear, and each percentage point of Dodge or Block will reduce damage by a equal amount. Parry, on the other hand, is almost as good as Dodge early on, when you have low amounts of Parry, but becomes less and less effective as you add more Parry.
(Please note that I am discussing raw Dodge and Parry, not Dodge Rating or Parry Rating. The amount of Rating per percentage point will make a big impact.)
In the next post, I'll look at the second Parry scenario. My initial instinct is that Parry will be much better under those rules. After all, in the second scenario, 100% parry would translate to 25% damage all the time.