Lethality: The Truth

Lethality is a 5-point rogue talent on the third tier of the Assassination tree. 5 points in Malice (5% increased crit chance) and 10 total points in Assassination are required before you can take Lethality. Straight from the game, we can see the description as follows:

(5/5) Lethality increases the critical strike damage bonus of your Sinister Strike, Gouge, Backstab, Ghostly Strike, Mutilate, Shiv, and Hemorrhage abilities by 30%.

What does this mean in simple terms?

5/5 Lethality makes your spammable special attacks do 15% more damage when they crit.

Lethality gives a max of 15% increase to crit damage, not 30%

Many players mistakenly think their crits are being increased by 30% due to the wording of the talent ingame. Due to the nature of crit calculation, however, a 30% increase to critical strike damage bonus equates to only a 15% increase in the actual damage of the attack.

By default, an attack that hits for 100 will crit for 200.
This 200 crit is made up of two parts, 100 base damage and 100 bonus damage
For 5/5 Lethality, we increase the 100 bonus damage by 30%, giving us 130 bonus damage
100 base damage + 130 bonus damage = 230 total damage
230 / 200 = 115%, thus our crit now does 15% more damage
LethalityIncrease = 15%

Gotcha #1 - the bonus only applies to critical strikes

While 15% sounds like a solid number, you must keep in mind that this bonus from Lethality only applies to critical strikes. Based on the critrate of our special attack, we can calculate exactly how much of the damage comes from critical strikes. A simplified formula for the proportion of crit damage for a rogue is:

CritFraction = 2*CritRate / (1+CritRate)

To show you why this simple formula makes sense, we can look at an example of Sinister Strike with a 25% critrate:

With a 25% critrate, we expect one in four attacks to crit
We'll use example attacks of 100, 100, 100, 200(crit)
Damage from crits = 200
Overall damage = 100+100+100+200 = 500
CritFraction = 200 / 500 = 40%

Now let's plug the same example numbers into our formula, where CritRate = .25

CritFraction = 2*CritRate / (1+CritRate)
CritFraction = 2*.25 / (1+.25) = .5 / 1.25 = 40%

Given the fact that we know Lethality will increase our CritFraction by 15%, we can multiply them together to get the total DPS increase for our special attack (Sinister Strike in this case).

SpecialDamageBoost = CritFraction * LethalityIncrease
SpecialDamageBoost = 40% * 15%
SpecialDamageBoost = 6%

Gotcha #2 – special attacks are only a small fraction of your total damage

To truly gain perspective on how much your damage is increasing due to Lethality, you need to determine how much of your total damage comes from special attacks (Sinister Strike in our example). This is not really a concrete number, but one that must be measured by your performance ingame. From my own experiences and input from several other rogues over time, I can provide fairly accurate estimates of this number for the sake of our discussion.

Basic estimate for any given build/gear level = 35%
Combat rogue, Raid setting = 25% - 35%
Combat rogue, PvP setting = 30% - 40%
Backstab/Mutilate rogue, PvP setting = 35% - 45%

Particularly with raiding rogues, this number will tend to get smaller and smaller as your gear improves, since auto-attack is the strongest-scaling component of your overall damage with +hit gear, haste, and attack power. In PvP scenarios, the number tends to be slightly higher due to the stop-and-go nature of PvP combat, which can reduce the relative contribution of auto-attack and increase the opportunities to unload a full energy bar.

We'll call this number the SpecialFraction, as it represents the fraction of our total damage that comes from our special attack. We can multiply this by our SpecialDamageBoost to determine the overall boost to our damage from Lethality.

TotalDamageIncrease = SpecialFraction * CritFraction * LethalityIncrease

For an example, we'll use a Combat Sinister Strike rogue with a CritRate of 25% and a SpecialFraction of 35%.

TotalDamageIncrease = 35% * 40% * 15%
TotalDamageIncrease = 2.1%

Lookup table (PVE) – find how much 5/5 Lethality is helping you

I have created a table of values that will help you estimate what kind of a DPS increase you are getting from Lethality in a typical PVE scenario (i.e. no resilience). To find your number, choose the CritRate of your special attack on the left side, and choose the SpecialFraction across the top.

Lookup table (PVP) – 5/5 Lethality vs. 400 resilience

This second table uses the same principle as the one above, but it uses a modified formula that factors in the presence of 400 resilience (~10% critrate reduction, ~20% crit damage reduction) on your target. This is a nice round number that approximates the resilience of an opponent with full arena gear.

The new formula for CritFraction takes into account the critrate reduction and reduced crit damage:

CritFraction = 80%*2*(CritRate-10%) / ( 1+(CritRate-10%)*(80%*2 - 1) )

To find your number, again choose the CritRate of your special attack down the left and your SpecialFraction across the top.

The bottom line?

The moral of the story is that you should really think hard before you put any points into Lethality if you play a Sinister Strike build, particularly if your focus is PvP combat. While it is viewed by many as a "must have" talent in the Assassination tree for any build, I hope that some of these numbers can convince you otherwise. Giving 5 points to this talent for what will usually give you less than a 2% DPS increase may not be your best option, and could potentially be traded for some talents with "intangible" qualitites in the Combat or Subtlety trees.

If you are only interested in PvE raid damage, you will still want points in Lethality because there are no other alternatives that boost damage output, and Mutilate rogues have so many points into assassination that it makes sense to grab it anyway.