Hey there! I did some testing in the recent PTS and these are my results. Some of it is extraneous but I wrote it previously but wasn't sure if it would be of interest to anyone. The answer to your question is like in the first few sentences. Hope it helps

Recently, a guild mate asked me whether he should put Reinforced or Impenetrable on his gear for PvP. To begin an understanding of how Impenetrable works is needed. A gold impenetrable piece claims to ‘Increase resistance to Critical hits by 250’. This does not seem very informative. My testing suggests that a gold impenetrable piece reduces the critical damage multiplier by approximately 3.5%. Thus without any pieces of impenetrable the base critical damage multiplier is 1.5 but with 2 pieces of impenetrable this is reduced to 1.43. As an aside this stacks additively with Resilient. With this information the analysis can continue.
Let the base damage without taking into account critical damage be, D. From my previous testing
D = Tooltip * Battle Spirit * Attacker CP * Defender CP*(1-Mitigation)
Attacker CP here includes Thaumathurge or Elemental Expert and Defender CP refers to Elemental Defender. The first four terms can be combined together into a single parameter, T, for this analysis. Mitigation is given by the following formula
Mitigation = (Base Armour + Reinforced Armour)*Penetration/66000 – Focus/66000
Mitigation = Base Armour*Penetration/66000 – Focus/66000 + Reinforced Armour*Penetration/66000 Mitigation = Base Mitigation + Reinforced mitigation
where I am assuming that both the attacker and target are V16. I’ve explicitly neglected additional penetration from Spell Erosion and Piercing.
Then
D = T * (1-Base Mitigation + Reinforced Mitigation)
Now let us consider the damage done while taking in account critical damage, D1.
Reinforced
D_reinforced = (1-C)*D + C*M*D
where C is the Critical Chance and M is the critical damage multiplier
D_reinforced = D (1 – C + C*M )
D_reinforced = T ( 1 – Base_mitigation – Reinforced_mitigation ) ( 1 – C + C*M )
Impenetrable
D_impenetrable = ( 1 – C ) * D + C * ( M – I ) * D
where I is the reduction of the critical damage multiplier.
D_ impenetrable = D ( 1 – C + C * ( M – I ) )
D_ impenetrable = T ( 1 – Base_mitigation) ( 1 – C + C*M – C*I )

Reinforced – Impenetrable

If (Reinforced – Impenetrable) is positive this means that Impenetrable is favoured. The converse is true.

Reinforced – Impenetrable = T ( C*I*( 1 – Base_mitigation ) – Reinforced_mitigation ( 1 – C + C*M))

From this, the following observations can be made
• Impenetrable is more effective at lower base mitigation (i.e. light armour wearers benefit more)
• A higher critical damage multiplier results in benefits only for reinforced

I’ll calculate (Reinforced – Impenetrable) for a range of Reinforced Armour and Critical Chance. To put into perspective the range of Reinforced Armour. The reinforced traits gives 16% increased armour for a legendary item. The lowest armour piece at V14 is a light armour sash with a base armour of 511. Thus for this particular item, reinforcing it would increase Reinforced Armour by about 82.
Using a critical damage multiplier of 1.5 and an extra reinforced mitigation of 100/66000 the following is calculated



This shows that impenetrable is always preferred when comparing between 250 increased critical damage resistance and 100 bonus armour from reinforced

Asayre wrote: »

This shows that impenetrable is always preferred when comparing between 250 increased critical damage resistance and 100 bonus armour from reinforced

yah so impenetrable right, in particular with light armor? Thanks for that amazing information...

I have about 20k health in Cyrodiil on one toon and a friend has about 33k. My friend has no impenetrable, I have at least 5 pieces at purple quality (cant recall the dropped armor traits). We pulled a district roaming boss and the friend about chain died by popping like a baloon, while I could eat it and heal up.

A stamblade popped on me at one point and did not do enough damage to pressure me into defense.

So anecdotally, impen is pretty decent now.

Wreuntzylla wrote: »

I have about 20k health in Cyrodiil on one toon and a friend has about 33k. My friend has no impenetrable, I have at least 5 pieces at purple quality (cant recall the dropped armor traits). We pulled a district roaming boss and the friend about chain died by popping like a baloon, while I could eat it and heal up.

A stamblade popped on me at one point and did not do enough damage to pressure me into defense.

So anecdotally, impen is pretty decent now.

Except mobs in this game do not crit. So your first example is just the placebo effect.

Here's another perspective / set of data: goo.gl/84hnar

http://forums.elderscrollsonline.com/en/discussion/231578/how-does-impenetrable-currently-work#latest