Proc Mechanics
[top]Proc Basics
Procs are effects that have a chance of occurring under a certain triggering condition. Typically this is used to mean effects that trigger off physical attacks or spellcasts; however, the term also can be used to apply to events that are triggered by being hit, such as the effect on
[Bulwark of Azzinoth] or
[Band of the Eternal Defender]. For purposes of this article I will be using the terminology of on-attack procs, but the principals here apply equally to procs of other sorts.
Fundamentally, there are two important things to know about a proc: its proc chance, and it's internal cooldown.
[top]Proc Chance
Procs generally fall under one of two categories. The first catagory are fixed-percent procs. No matter what weapon is used to trigger these, the chance that they will proc is the same. For instance, a Rogue's Combat Potency ability has a 20% chance to proc on all off-hand hits, regardless of the speed of the weapon used. If you attack once a second or once every 5 seconds, you will still proc on 20% of attacks. Similarly,
[Sextant of Unstable Currents] has a 20% chance to proc on any spell critical strike, regardless of what spell is used.
The second category is referred to as Proc Per Minute or "PPM" effects. These are normalized such that if you autoattack with a single weapon, the average number of procs you get in a minute does not depend on the speed of the weapon you use. So, as a simple example: Mongoose is thought to be a 1.2 PPM effect. Hence, if I autoattack with a 2.0 speed weapon, it will proc on 4% of attacks, such that over the course of a minute (during which I will make 60/2.0 = 30 attacks) it will on average proc 30 * .04 = 1.2 times. If, on the other hand, I autoattack with Mongoose on a 3.0 speed weapon, it will instead proc on 6% of attacks, such that it will still proc an average of 1.2 times per minute (60/3.0 = 20; 20 * .06 = 1.2).
It should be noted that for any proc effect implemented in the Burning Crusade era (post-60), any haste effects in play are counted against your weapon speed for purposes of autoattacking. For instance, if I have 25% haste on, I will attack 25% more times in a minute, so each proc will be correspondingly less likely to proc such that I still average the same number of procs in a minute. Note that this is *not* true of pre-TBC proc effects such as Crusader - on these older effects, the chance of a proc going off is purely a function of your base weapon speed.
Note, however, that this applies only to autoattacks. If one is additionally launching instant attacks - Sinister Strike, Mortal Strike, or whatever else - these have a chance to proc as well, and can thus increase the number of observed procs. Unlike autoattacks, haste effects do not reduce the proc rate of instant attacks; if you are using mongoose on a 3.0 speed weapon, you will have a 6% chance to proc no matter how much haste you have on.
At this point an example would probably be helpful. Consider, for instance,
[Madness of the Betrayer]. This is a trinket with a 1 PPM armor penetration proc. Let us assume for the moment that I am attacking with a 2.5 speed MH and a 1.5 speed OH. Let us further assume that I have 50% haste from various sources, and that I use Sinister Strike once every 4 seconds, and have WF totem. Then my MH autoattack occurs 36 times a minute, so each MH autoattack has a 2.78% chance to proc Madness of the Betrayer. However, those 36 attacks also proc an average of 7.2 WF procs per minute, which are considered autoattacks, so also proc at this rate. Thus, in reality I will generate (36 + 7.2) * 2.78 = 1.2 procs each minute from autoattack and Windfury. Now, my OH attacks can *also* proc Madness of the Betrayer, but since I average 60 attacks per minute with that weapon, each only has a 1.67% chance to proc. Thus, my OH generates 1 proc each minute on average as well. Finally, I make 15 instant attacks with Sinister Strike, each of which procs with a 4.17% chance (as the proc rate is note reduced by the haste effect in play). Thus, I get an additional 4.17 * 15 = .625 procs each minute from instant attacks. Thus, in the average minute, Madness of the Betrayer will proc 1.2 + 1 + .625 = 2.825 times each minute.
[top]Internal Cooldowns
Some trinkets additionally have an internal cooldown - that is, after a proc occurs, there is some duration that must elapse before the trinket can proc again. For instance, when
[Band of the Eternal Champion] procs, it cannot do so again for 60 seconds. No two procs can ever occur less than 60 seconds apart. This is also an important aspect in determining the utility of a proc. It should be noted that all known procs either have an internal cooldown of 0 (that is, no cooldown at all) or an internal cooldown longer than the length of the buff. This fact will be important later.
What's really important in figuring out the usefulness of a typical buff is the uptime of the buff - that is, what portion of the time that buff is in effect.
The easy case is addressing what happens when there is an internal cooldown longer than the length of the buff. If we define
D = Duration of the buff
p = Chance the the buff procs on any given attack
v = Number of attacks that can trigger the proc that occur in a given second, and
c = Internal cooldown of the trinket
U = uptime, then
The harder case is what happens when the proc does not have an internal cooldown. One might expect that if one had, say, a 1 PPM effect with a 15 second duration, the uptime would be 25% (15 seconds uptime per proc, one proc per 60 seconds, 15/60 = 25%). One would, alas, be incorrect. Because there is no internal cooldown, the procs can overlap, which reduces the actual uptime observed. The correct formula (using the same variables as above) is:
^{Dv})
[top]Known proc rates
A brief listing of procs rates of commonly-used effects:
- Mongoose: 1.2 PPM
- Executioner: 1.2 PPM
- Hourglass of the Unraveler: 10%, 45 second internal cooldown
- Dragonspine Trophy 1 PPM: 20 second internal cooldown.
- Tsunami Talisman: 10%, 45 second internal cooldown
- Warp-Spring Coil: 25%, 30 second internal cooldown
- Madness of the Betrayer: 1 PPM
- Band of the Eternal Champion: 1 PPM, 60 second internal cooldown.