Last time, I explained which stats protect you and how, in this post, I'll be explaining how they interact with eachother, and what stats you should stack in your tanking gear.
The three basic evasion abilities, parry, dodge, and miss chance, stack additively. That is to say, 5% miss chance, 20% parry, and 25% dodge means only 50% of enemy attacks will hit you, not 57% if it were multiplicative like most people think.
The conventional wisdom that most people assume is that the game first rolls to see if an attack would miss, (5% miss, 95% continue on in the above example) then of those that do not miss, the game might check to see if those get dodged, (5% miss, 23.75% dodge, 71.25% continue) then of those neither missed nor dodged, it would roll parry (5% miss, 23.75% dodge, 14.25% parry, 57% hit).
But this is not how it actually works. Instead, they stack additively, making your chance to parry, dodge or be missed exactly the listed number on your character sheet. So this leads to the question of what happens if the numbers add up to more than 100%. If the total chance exceeds 100%, things start falling off the bottom of the attack table.
The attack table for melee attacks looks like this:
Glancing Blow (Usually irrelevant, only when a player attacks a higher levelled mob)
Block (Irrelevant for Death Knights)
Critical Hit (Irrelevant when defense capped)
Crushing Blow (Usually irrelevant, only when a mob 4+ levels above the player attacks)
Without the irrelevant lines, your incoming hits should look like this:
So, what implications does this have? If for some reason your Miss + Dodge + Parry is greater than 100%, nothing will ever hit you with a melee attack. If Miss + Dodge + Parry + Chance to be Crit is greater than 100%, the mob will never land a regular hit on you, only crits. This applies to your outgoing attacks also, by the way, so it is possible to have so much crit that you are crit capped, because there isn't any more room at the bottom of your table to fit crit into, in which case you would need more hit and expertise rating.
For spells attacks, the table is a lot smaller:
Miss (Which is displayed by the game as Resist)
And ranged attacks:
Crushing Blow (Irrelevant: Only when attacked by a mob 4+ levels over the player)
So what this means is parry and dodge are useless for mitigating ranged and magic attacks (excepting Spell Deflection).
So how does Spell Deflection work? After the spell hits (or crits) it rolls to see if your spell deflection procs, and then ruduces the damage as appropriate. This is typically how all damage reduction effects work, and they stack multiplicatively, so order does not matter. As an example, if spell deflection reduces a hit by 45%, and Frost Presence reduces it by another 5%, then the actual hit does 52.25% damage. Armor and resistances are also applied in the same way.
After rolling to hit and applying damage reduction, then shields (such as Power Word: Shield or Anti-Magic Shell) take damage, and any leftover damage beyond the shields' capacity hits the player's health.
I do not know when non-percentage damage reduction effects (like Unbreakable Armor or Lesser Absorption) take place, but I may run an experiment to test it at some point. Currently, my guess is that they apply before shields, but after percentage reduction.
Now, for a word on a different kind of stacking. You are probably familliar with the concept of item levels, and that higher level items are usually, but not always, better than lower levelled items.
When the designers creat an item, the level assigns the item a certain budget of item points, which are used to 'buy' stats for the item. The price of a stat varies by which stat it is, and by how big the stack of that stat on the item is. As an example, an item with 100 stamina would have the same item level as an item with two separate stacks of 66 stamina and 67 stamina (although usually you don't see multiple stacks of the same stat on an item, it does happen.)
As a rule of thumb, you can get more stats per item level if the stats come mostly in smaller numbers and if there are more separate stacks of stats on the item. This is because the price of a stack is multiplied by the size of the stack raised to the 1.7025 power. So 100^1.7025 is approximately equal to 66^1.7025 + 67^1.7025. So if you were to find an item that would otherwise be useful, but has maybe 30 points of Haste Rating, (which Death Knights do not care about,) worry not, those points are nearly free by virtue of being in a small stack, and should not prevent you from wanting and rolling for the item.
Note, that since you have complete control over which gems and enchants go on your gear, you should not care about what item level an enchant or gem would be worth, and only care about the effect you get out of them.
And one more type of stacking:
You may have noticed that Blessing of Wisdom and Mana Spring Totem will not work at the same time. That is because of one last type of stacking to confuse with the others. For each stat that you could concievably boost in some way, it can only be boosted once by each type of boost. For any buff you have, it falls into one of several categories.
Spell buff is the category the above example would have fallen into. Blessing of Wisdom will stack with an Elixir of Mighty Mageblood, because it is a Guardian Elixir, but not with Mana Spring Totem because it is also a spell buff.
The categories that I know of are:
Battle Elixir (limit 1 battle elixir)
Guardian Elixir (limit 1 guardian elixir)
Flask (will not stack with any elixirs) (limit 1 flask)
"Well Fed" Food Buff (limit 1 food buff)
Talents (stacks freely with eachother)
Equipment (stacks freely with eachother)
Movement Speed Buff (irregardless of the source, all movement speed buffs fall into a single category except mounted speed buffs, one at a time of which will stack with a mount. Movement buffs on items that are overwritten will not be removed from the item, but will stop working until the new buff goes away.)
Mounted Movement Speed Buff
Movement Speed Debuff (stacks additively with a movement speed buff. Ex: 15% slow + 10% haste = 95% speed overall.)
...and there may be others.
When multiple buffs of the same type come into conflict, one of a few things will happen, but the most powerful buff for that particular stat will always be the one that remains.
If a new buff is more powerful than the other, it will overwrite it. Casted buffs will not dispel an aura/totem, but will stop it from working on the affected player.
If a new buff is weaker than the other, it cannot be cast. For DK tanks, this can be problematic when a shaman has (improved) Strength of Earth active, you may find yourself unable to even cast Horn of Winter, preventing you from using it to generate runic power.
And finally, if the buffs are of equal strength:
If both are the casted, the spell with the longer duration remaining will be considered more powerful.
If both are auras/totems, both will be active, but only one will affect each player in range.
So, hopefully now you know way more about stat stacking than you ever wanted to, and can properly understand why 1% parry is way more effective when you have 30% dodge than if you had 5% dodge.