Damage over time abilities are largely measured by their DPET, damage per execution time.
Lets say you have a DoT that lasts 15 seconds and ticks every 3 seconds. You have no haste at all and your execution time is 1.5 seconds for the base global cooldown. If it does 3,000 damage per tick for a total of 15,000 damage, the damage per execution time of the ability is 10,000 (the execution time of an instant is the global cooldown). If you clip the last tick to recast the spell, your prior cast has had its total damage reduced by 3,000, making the damage per execution time 8,000.
The actual contribution of this ability towards your total DPS has been reduced by 20%, because you wasted 20% of the damage that would have been done. In actuality, it actually costs you more than this because of the time since the last tick also being wasted. You not only pay the loss of damage of that tick, but you pay the loss of damage of the time passed leading up to that tick. In this simple case, 20% loss is a theoretical minimum. If you delay 2.9 seconds before cliping the DoT for example, the actual amount of damage per execution time wasted is 5.9, the loss is 5,900 damage towards your damage total in the damage per execution time, or lowering the DPET of your ability to 6,067, almost a 40% drop. You only actually lost 3,000 damage for the one tick, however, you lost more damage in the time you wasted. You didn't just waste the damage of the tick, but the time leading up to the tick.
This ability does 1,000 DPS (15,000 damage over 15 seconds) added to your other spells while you are casting. So effectively while this debuff is actively running, you increase your DPS during that timeframe by 1,000. If there is a delay of say 1.5 seconds before you renew your DoT after it expires (you can expect to spend at least ong global cooldown to make sure not to clip your DoT), you deal 15,000 damage every 16.5 seconds with it, so it contributes a total of 909 towards your total DPS for the fight. Now, if we clip the DoT and lets say you are perfect and renewed it at exactly 12 seconds. Your DPS contribution of this ability towards your total DPS is 1,000 because it is never down and never any wasted time leading to the next tick. Where is the loss you ask? You haven't reduced the DPS of the DoT any, but you have decreased the damage per execution time of it. Now here is how that plays in. On that 12 second cycle, you spent 1.5 seconds on a GCD at the start for putting up the DoT. Now lets say you case 7 1.5 second cast direct damage abilities that deal an average of 6,000 damage. 6,000 / 1.5 is 4,000 damage per execution time of your spammed spell, and 4,000 DPS if you only spammed that spell. 6,000 * 7 + 12,000 is how much damage you deal in your 12 second rotation with cliping of your DoT. That is 54,000 damage, or 4,500 DPS over the 12 second rotation. Now back to the first case with letting the DoT run the full duration but having a 3 second delay in renewing it (16.5 second long rotation). That one has 6,000 * 10 + 15,000 or 75,000 total damage over 16.5 seconds for 4,545 DPS. You increased your DPS by 1% by not clipping your DoT, even when you were absolutely perfect at clipping your DoT at the ideal moment. If you cast one additional spell on your clipping rotation for 6,000 * 8 + 12,000 over a 13.5 second interval, because of the additional 1.5 seconds of the last DoT tick wasted you would drop to 4,444 DPS (down another percent because of the partial tick wasted).
Even if you can clip your DoTs perfectly, it will net you a loss in DPS, not because the DoT itself decreased in DPS, but because the damage per execution time decreased.
You only clip to renew DoTs early if...
1. There is a fight mechanic that prevents you from continuing DPS such as Putricide phase change or Icehowl knockback.
2. You are forced to move and don't have alternative instant cast abilities which can be used during movement.
As a very general point of view...
DoTs tend to have higher damage per execution time of direct damage abilities.
Direct damage abilities have higher DPS than DoTs.
There are exceptions to this general rule, for instance the Eclipse buff for a Balance Druid greatly alters the DPET of the direct damage casts such that they may overtake the DPET of the DoTs.
Because DoTs have higher DPET, they are higher priority casts because they will in the end deal more damage for the time spent casting it than a direct damage ability, but it takes time to deal that damage. Inbetween those high DPET attacks you favor your higher DPS attacks, direct damage attacks where DPET is directly equal to DPS. In the example above, that 4,000 DPS ability only did 4,000 DPET where as the DoT did 1,000 DPS but 10,000 DPET. The higher the DPET of an abiity, the higher it is in your priorities to use it. That's why when DoTs are down it is typically a high priority to renew them quickly.
Last edited by Quinafoi; 05-28-2010 at 08:24 AM.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.