EDIT: See Part 2 for a more in-depth discussion.
Q: WTF is up with this armor penetration stat?
A: Most of your character’s attacks are affected by your target’s armor. Armor Penetration enables these attacks to ignore a certain percentage of armor.
Q: So, like, which attacks?
A: Generally, direct attacks are reduced by armor (and thus increased by ArPen), while DoT effects, such as kitty bleeds, are not.
Q: That doesn’t seem too complicated. Why all the bitching?
A: Because it scales in value dynamically, compared to other stats which scale in value linearly.
A: Ok, let’s explore some other stats, then come back to ArPen. Bring in Example Kitty! Here are Example Kitty’s unbuffed stats, which are based on a character snapshot I took in late 3.1 (FWIW, Naxx-25/Uld-10/Uld-25 gear, no hardmodes), and fed into Toskk’s Cat DPS Calculator. (Available at work!)
- Str: 248
- Agi: 1355
- Hit: 236
- Haste: 226
- AP: 7674
- CritRating: 702 (54.03%)
- Expertise: 38
- ArPen: 204
- Talents: All
- Glyphs: Shred/Rip/SR
- Buffs/Debuffs: None
- Bonuses: Relentless Earthsiege meta, Mongoose chant, Idol of the Corruptor, 2pc T7, DM:G trinket, Mirror trinket.
Settings: Default, except all OoC procs for Shred checked.
According to Toskk’s Cat Calculator, I should be able to pull 4407 DPS with these stats. Let’s look at AP, to start. AP scales linearly, so here, with every point of AP I add, I gain 0.363 DPS. No matter how much AP I gain (or lose), if all my other stats remain the same, each point of AP is worth the same amount.
For something a little harder, let’s look at Hit. Hit scales linearly as well, but has a cap. Currently, 1 point of hit is worth 0.597 DPS to EK, and that stays the same, regardless of whether i have 0 hit or 261 hit. Above 261, (or to be exact, 262.3), hit is worthless, since you can no longer miss a raid boss.
Now, finally, ArPen. ArPen works much differently, in that the value of individual points starts out very small, but increases as you accumulate more, until it finally hits a cap of its own. This cap is currently at 1230, and will be increasing to 1398 in 3.2.2 (more on this in a sec).
So, keeping EK’s other stats current (3.2.1/3.2.2 values):
With 0 ArPen, 1 point is worth 0.57/0.50 DPS;
With 300 ArPen, 1 point is worth 0.67/0.58 DPS;
With 600 ArPen, 1 point is worth 0.81/0.68 DPS;
With 900 ArPen, 1 point is worth 0.99/0.81 DPS;
With 1200 ArPen, 1 point is worth 1.25/0.98 DPS.
Remember that this is unbuffed; buffs/ArPen trinkets/debuffs can change the math significantly (see next Q)
Q: And these numbers are important because…
A: Well, they’re only relevant in context with Agility. Normally, we want to stack +agi over everything else, as +agi gives us our biggest boost per point. However, at a certain ArPen/stats level, +ArPen becomes more valuable. Let’s run a few more examples:
Case 1: EK, unbuffed as before.
Case 2: EK, 10-man buffs/debuffs (Flask of E. Rage, F. Feast, Imp. MOTW, Horn of Winter, Heroic Presence, BoKings, Sunderx5, FF)
Case 3: EK, 25-man buffs/debuffs
(Same as Case 2, add Imp. BoMight, Windfury, Unl. Rage, Bloodlust, F. Inspiration, Imp. Moonkin Form, Heart of the Crusader)
Case 4: EK, 25-man buffs/debuffs, swapping in Grim Toll for Mirror of Truth.
The following data tables/graphs are measuring the effectiveness of stacking +ArPen over +agi, measured by percent effective. (For instance, a result of .75 means that adding ArPen, in this particular case, has 75% the effectiveness of adding agi.)
|ArPen 3.2.1||Case 1||Case 2||Case 3||Case 4|
|ArPen 3.2.2||Case 1||Case 2||Case 3||Case 4|
As you can see, 3.2.2 really kills off the idea of ArPen stacking for lower gear levels, with your average 25-m raid requiring ~900 ArP from gear in order for stacking it to be an improvement. Remember, all of this will scale with gear, so feel free to enter your own numbers into the calculator.
Q: TL;DR. Sum it up for me!
A: Sigh. Okay. Assuming you have ~ivl 215 gear, as it stands now (3.2.1), you want to switch to gemming/flasking/eating for ArP around 450 for 25m raids, or 300 if you have Grim Toll/Mjolnir’s Runestone. (high 600′s for 10m raids.)
For 3.2.2, you would switch at 850ish for 25m raids, and 1150ish for 10m. For GT/MR, I need to test it some more and get more data points, but at this gear level, the proc is tough to manage since you have to (attempt to) stack so much ArPen for it to be useful, but then much of the proc effect is wasted due to the cap.
In short: gem/flask/eat for agility.
EDIT: See Part 2 for a more advanced discussion at higher gear levels…the decision scales not only off your personal ArPen, but your other stats as well.