How much healing is PW: Barrier worth?

Barrier is one of the great new additions for Cataclysm, initially so good that Discipline priests were almost mandatory for much of T11 heroic modes. It's since been nerfed a bit (well, a lot actually) but it's still a mainstay of heroic raiding because encounter design tends towards lots of damage concentrated in short periods rather than being evenly spread through a fight.

So it's good, but how good? Is it as good as Tranquility? Better?

It's a difficult question to answer actually. Cataclysm also brought as one of its changes logging of absorbs in the combat log, something that was sorely missing and previously only guessed at by parsing tools like Recount or World of Logs. But mitigation abilities, those that reduce damage by x%, don't show up (with the exception of resists), all that's logged is the damage taken with no clue as to how much it was reduced by. Further complicating things is that you can't assume a flat percentage for the ability you're interested in because damage mitigation effects are multiplicative such that as you have more active at once, each is weakened somewhat. From a recent discussion I had over at the excellent How To Priest forums I had this example of how talenting 2/3 Inner Sanctum wasn't worth the headline 4% because of how shadow priests already benefit from Shadowform:-

You get hit for 100k damage
You take off 15% for Shadowform ie 15k
You take off 4% of the remaining 85k for Inner Sanctum, ie 3.4k
Combat log shows damage of 81.6k (100k - 15k - 3.4k), for an 18.4% reduction overall

You don't have to be a genius to see that 4% then becomes 3.4% or (1-0.15) * 4. You could look at it another way and say that Inner Sanctum is devaluing Shadowform instead, but the numbers work out the same.

You get hit for 100k damage
You take off 4% for Inner Sanctum ie 4k
You take off 15% of the remaining 96k for Shadowform, ie 14.4k
Combat log shows damage of 81.6k (100k - 4k - 14.4k), for an 18.4% reduction overall

15% becomes 14.4%, or (1-0.04) * 15. How you fairly attribute this is something I'm still thinking about, because my math skills are lousy, but I imagine it'll involve splitting the difference between the two abilities in a weighted manner.

Also, magic damage is often capable of being resisted and physical damage is reduced by your armor. So you see a boss ability in the tooltip state some amount you're sure looks like a raid wiping number, but in reality you see much less of that, so Barrier isn't worth 25% of the tooltip amount. Fortunately resists are logged but I don't know at what point you roll for resists without testing. Unfortunately there is, as far as I know, no way to know someone's armor value from the combat log, which makes physical damage a little more tricky to get figure out for the purposes of Inspiration or Ancestral Healing.

Starting to sound like if only we could take account of all this stuff we could indeed work backwards from the combat log and find out how much we really did with Barrier, perhaps even throwing in some numbers from the glyphed healing bonus. What we need is something that'd parse the combat log, pull out what abilities and talents are active when, account for absorbs and resists appropriately and then fairly assign values to all mitigation effects. And I'm working on it. I've never coded before, but with a little testing in-game to work out a few things about the combat mechanics and logging, I'm pretty confident I can scrabble together enough Lua to give an accurate answer. I know what I need in psuedo-code and how hard can it be to google the rest...?

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