Big pulls with Dark Knight.

[Kabooa]

Please ELI5 it for me xD

EHP stands for Effective Health/Hit Points.

It is how much "Raw" damage has to go out to kill you.

Keep in mind you almost never see "Raw" damage beyond very early in the game, as by the time you are level capped, based on your job, you have a certain level of defense, and in the case of tanks, a trait. Your "HP" at the bottom, the green bar, is not your "EHP". Tank "HP" goes far beyond what equal amounts of DPS "HP" does.

So lets say you're a black mage and you have 100,000 HP. Your defense score at level 80 end game armor will put you at approximately 18-20% mitigation. This means you take 80% damage, or .8. You find your effective hp by the following.

HP / Damage Taken. 100,000 / .8 = 125,000 EHP.

However, since we never don't have our defense, this is considered our "Baseline". Our 100%. The damage we see when we get hit, for example, 7500, is already taking our baseline mitigation into account, and is not "Raw damage". For all practical purposes however, it's 100,000 health vs 7,500 damage.

Your understanding of the end result of stacking mitigation is correct.

However this is not "Diminishing return". Diminishing return deals directly with Cost and Benefit. The End result is not the "Benefit", because the end result has more factors.

The benefit of more mitigation is higher EHP values.

In order for mitigation to suffer from diminishing returns, each additional layer needs to be penalized in its benefit, but it is not.

With 20% mitigation, 100,000 HP becomes 125,000 EHP. A 25% increase.
With 30% mitigation, 100,000 HP becomes ~143,000 EHP. An ~43% increase.
With both layered together, 100,000 HP becomes (100,000) / (.8 * .7 = .56) = 178,571, or about a 78.6% increase.

This effectively means that Every layer of mitigation in a Multplicative System is exactly as effective as it would be alone compared to if they are used together.

The above example provided by Rei is flawed because it paints a very specific scenario, despite being vaguely described.

"The tank is taking 5k DPS over 40 seconds, all damage intake is uniform".

My counter example is no less specific.

"The tank is taking 280k Damage over one attack"

This demonstrates that "Diminishing return" is not about how much damage is prevented. That is separate, and we can go back and forth on the validity of each example, but in the end it has nothing to do with the concept.

It's about Cost (The ability) and Benefit (The EHP increase), and in multiplicative systems, the Cost and the Benefit do not fall out of sync.

This does not mean you cannot use abilities unwisely. You still gain the benefit - But you just didn't need it.