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http://taugrim.com/2012/01/19/unders...-tanks-in-pvp/

Copy of the article in full below.

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This article describes the Defense and Shield mechanics for SWTOR and their implications for tanks in PVP.

Defense is the Avoidance mechanic and Shield is one of the Mitigation mechanics. In this article I use the terms "defense" and "avoidance" interchangeably.

Avoidance and Mitigation in SWTOR

To quickly define terms:

Avoidance: not being hit
Mitigation: reducing damage taken when hit

There are 3 categories of Avoidance as captured in the Defence Chance tooltip:
Melee Dodge: base of 5%, boosted by Defense Rating, talents, and buffs
Ranged Dodge: base of 5%, boosted by Defense Rating, talents, and buffs
Resistance: base of 0%, boosted by talents and buffs. This is against "spells" (Force and Tech attacks)

Mitigation comes from multiple sources:
Expertise: stat that provides mitigation against all damage types
Armor: mitigation against 2 of the 4 damage types: Energy and Kinetic. Keep in mind that for attacks that deal "weapon damage", to understand whether Armor factors in, you have to check the damage type of the weapon itself. E.g. blasters and rifles deal Energy damage

Talents that provide flat damage reduction
Shield Chance / Shield Absorption: provided when you have a Shield offhand equipped and successfully Shield an attack that hit. The amount of damage mitigated (absorbed) is based on the offhand (e.g. 20%), your Absorption Rating, and talents in the tanking tree. SWTOR's Shield functions similarly to WoW's Block mechanic. The main difference is that in SWTOR you don't have to keep oppponents in your frontal facing to Shield
"Bubbles": mitigation against all damage types
The 2-Roll System

SWTOR leverages a 2-roll system for determining the outcome of an opponent's attack as explained by BioWare's Georg Zoeller (references: 1 2), bold emphasis mine:


Quote:
First is a hit roll, accuracy versus defense, and if the attacker misses then no damage occurs. If the attacker rolled poorly enough to miss even discounting the target's defense then a "Miss" result occurs. If he misses because of the defense then the result varies based on the attack type, the cover state of the target, and the target's equipped weapons. All the possible results - Dodge, Parry, Deflect, Resist, Cover - are mathematically the same, but they can trigger different effects and are visualized in different ways.

If the attacker hits, then a second roll is made with the crit chance of the attacker versus the shield chance of the target. If a Crit or a Shield occurs then the damage is adjusted up or down (based on Surge/Absorb), and then it goes through to the armor and damage resistance. A critical can never be shielded, and an attacker with a high enough crit chance can push the target's shield chance off the table. It shouldn't be possible to get your passive crit chance high enough to start pushing off the target's shield chance, but there are short-duration buffs that push these chances high enough to come into conflict.

Which Attack Types are Defensible and Shieldable

There are 4 attack types: Force, Melee, Ranged, and Tech.

The attack types should not be confused with the 4 damage types: Elemental, Energy, Internal, and Kinetic.

Melee and Ranged attacks are Defensible and Shieldable, whereas Force and Tech attacks are not. This is consistent with other games such as WoW and RIFT, where "spell" attacks are not dodgeable / parryable and not blockable.

Implications for Tanks in PVP

A critical consideration for SWTOR is that all classes have either Force or Tech attacks. You can see the attack type by looking at the Abilities tab for a given character and looking at the right-hand column. Therefore, an implication is that you may not get as much mileage out of your Defense and Shield-related talents as you might expect. E.g.
Stun-based abilities across all Advanced Classes are classified as either Force or Tech attacks, so the target's defensive capability doesn't come into play
I play Bounty Hunter and Trooper and most of my key abilities are classified as Tech
Keep in mind per Georg's post that the attacker's Crit Chance can push Shield Chance off the table. There are a lot of buffs (e.g. talent-based, from Relic, etc) that boost Crit Chance, and the implication is that when you most need to Shield heavy incoming damage, your Shield Chance may be reduced significantly or completely. Given that, and the fact that Shield Chance is calculated after the hit roll, Shield Chance is not a strong mechanic in PVP.

Another implication is that the talents for boosting mitigation may be stronger than we might have expected. In particular, this explains why in my experience Guardians and Juggernauts tanks take the longest time-to-kill: they have multiple bubble mechanics in their tank tree, including Invincible (Warding Call) and Sonic Barrier (Blade Barrier), and these excellent abilities provide mitigation against all damage types.

I shared some of the thoughts in this article on a thread for my Guide to Powertech Mechanics and PVP a week ago, and some 50 Powertechs including chainsawsamurai tested and confirmed that the Avoidance and Shield-related talents do not work against Force and Tech attacks.

When I run as a tank-spec'd Vanguard, I will likely use a DPS PVP set based on the game mechanics and avoid (no pun intended) the talents related to Melee Defense, Ranged Defense, and Shield Chance, unless they provide some additional benefit (e.g. the Ammo-regen effect from Shield Cycler).

This is not to say that you shouldn't take the talents and tank PVP gear. If you want to max out your survivability, invest in them. It's just important to understand what benefit you get out of them and the mechanics.

Acknowledgements

Huge thanks to Kjollborn, who helped me to realize that the attack type not the damage type is what is factored into Melee Defense, Ranged Defense, and Shield Chance.

Thanks to Lightweight, a 50 Combat Medic Commando in my guild, for helping me with testing.