This is a pretty important concept. Here we show how armour and hitpoints are dependent upon each other in the grand scheme of things, and how they interact with each other. Always remember that you need to balance what you do with your armour and hitpoints against avoidance. And, of course, this entire thing is meaningless against spells. This is directly related to the theory of Effective Health, which examines this relationship in a slightly different way.
We can equate 1 point of stamina to a number of points of armour that will give you equivalence. That is, X points of armour will increase your time to live by the same amount as 1 point of stamina. The caveat here is that this applies only for melee damage; as soon as someone starts slinging spells it all means nothing, making this a mostly academic exercise. First, we note that the armour dynamic is a simple asymptotic function. In beta, mitigation was capped at 75%, and you can prove to yourself that it is still there by going and fighting much lower level opponents in release.(If you could break the 75% mark, you would achieve 98.96% mitigation at 1,000,000 armour against a level 70 opponent). Here's the analysis using 10000 armour (A) and 10000 health (H), facing a level 70 opponent.
For the formula to determine mitigation (M) for a given armour rating (A), we begin with:
K = 467.5L - 22167.5
Where L is the level of the opponent being fought, then:
A M = ----- A + K
We get 48.6% mitigation against a level 70 opponent with 10000 armour. Since we'll need it later, solving for A in terms
of mitigation gives:
MK A = ----- 1-M
Let's define survivability in terms of how long a given opponent takes to kill you. Assume we have 200 damage per second incoming, unmitigated, melee only.- Taking into account 48.6% mitigation, that's 102.7DPS.
Therefore, at 10000A and 10000H, 19.8 stamina is equivalent to 408A, or 1:20.6 stamina-armour equivalence, in terms of survivability under the given damage output.
Solving this for several other armour values while keeping health constant at 10000 and 15000:
10000H 15000H 8000A 1 stamina to 19A 1 stamina to 12A 10000A 1 stamina to 21A 1 stamina to 14A 12000A 1 stamina to 23A 1 stamina to 15A 15000A 1 stamina to 26A 1 stamina to 17A 20000A 1 stamina to 31A 1 stamina to 20A 25000A 1 stamina to 36A 1 stamina to 24A 35000A 1 stamina to 46A 1 stamina to 30A
Changing health total while keeping armour constant at 10000 and 15000:
10000A 15000A 7500H 1 stamina to 27A 1 stamina to 34A 8000H 1 stamina to 26A 1 stamina to 32A 10000H 1 stamina to 21A 1 stamina to 26A 12000H 1 stamina to 17A 1 stamina to 21A 14000H 1 stamina to 15A 1 stamina to 18A 16000H 1 stamina to 13A 1 stamina to 16A 18000H 1 stamina to 11A 1 stamina to 14A 20000H 1 stamina to 10A 1 stamina to 13A
We see that this equivalence is variable on both health and armour, but also that increasing health increases the value of armour very quickly, while increasing armour increases the value of health at a slower rate. Either way, overloading on one over the other drastically increases the value of the lower one.
One of the fundamentals of the calculation above is 48.6% mitigation against a level 70 opponent. Against raid level targets, which will be higher than level 70, your mitigation starts to go down. The effects are shown here:
Level 10000A/10000H 13000A/10000H 10000A/13000H Level 70 1 stamina to 20.6A 1 stamina to 23.6A 1 stamina to 15.8A Level 71 1 stamina to 21.0A 1 stamina to 24.0A 1 stamina to 16.2A Level 72 1 stamina to 21.5A 1 stamina to 24.5A 1 stamina to 16.5A Level 73 1 stamina to 22.0A 1 stamina to 25.0A 1 stamina to 16.9A Level 75 1 stamina to 22.9A 1 stamina to 25.9A 1 stamina to 17.6A Level 12000A/12000H 15000A/15000H Level 70 1 stamina to 18.8A 1 stamina to 17.0A Level 71 1 stamina to 19.2A 1 stamina to 17.4A Level 72 1 stamina to 19.6A 1 stamina to 17.7A Level 73 1 stamina to 20.0A 1 stamina to 18.0A Level 75 1 stamina to 20.7A 1 stamina to 18.6A
Finally, we can do some solving to get a general formula for stamina-armour equivalence against a given level opponent:
10(K+A) 1 stamina: ------- points of armour H K+A 1 health: ----- points of armour
Where A is your current armour, H is your current health, and K is as defined previously.
A final note is that this formula is not affected by the actual rate of incoming damage per second. As such, it is not affected by avoidance. An avoidance total of 20% from the combined dodge and parry of an individual character does nothing more than reduce the rate of incoming DPS by 20%, on average. While a 20% reduction of incoming DPS is significant (as is the reduction in critical hits that comes with it), it has no bearing on the stamina-armour equivalence.
So, what should you aim for when choosing gear? What we see here is that balance is pretty key to the whole thing. If you stack one at the expense of the other, you are doing yourself more harm than good overall. Having a good balance between A and stamina tends to focus both for you - when you are taking physical damage. Empirically, the 12000A/12000H pair is thrown around as a "good starting point" for Karazhan raiding. We see that this pair tends to be around 1:20. Magical damage is a factor that helps to balance the difference between higher health and higher armour. Having very high armour at the expense of health might seem to be of benefit against pure melee opponents, but is a definite disadvantage to opponents using magic, and the amount of buffer you have against hard hits and mishap is probably going to be too small - you can't stop every crushing blow. Your mileage will vary.