MajorJuggler and I have had our differences, but the way he calculates ship durability is--if you're careful to keep in mind his assumptions, which he's explicit about--rock-solid. But people aren't thinking about some of the conclusions they can gather from it, even without the burden of looking at the rest of MJ's work.
(I'm relying heavily on, and quoting from, the "Calculating Expected Durability" section of his post here.)
Let's take a random example: why is BBBBZ so good? Well, if you add up the normalized durability of that list, you find out that it's as tough to take down as 6.7 TIE fighters. It brings four ships with 3 Attack, plus a ship with 2 Attack to lead off or clean up, while rivaling the classic Howlrunner swarm for tankiness. Okay, then.
Another example: why are so many people struggling to make dual-Aggressor lists work? Well, it does narrowly edge out the YT-2400 as the most durable ship in the game, but while it costs as much as three Headhunters, it doesn't hit that mark in durability. In fact, a pair of Aggressors looks like a tank list, but they're only about as survivable as five Headhunters or TIE fighters. So if you're spending 28 points emphasizing the ship's offense, you think you're getting a tank list and ending up with a bit of a glass cannon. Look at shoring up the ship's defense, or at adding a Headhunter or Scyk, if you want the list to perform the way most people seem to think it will.
Why have "Three Amigos" lists like Wedge-Luke-Biggs always struggled, on the Rebel side? Well, they're uncomfortably fragile, is why: you can't name any competitive list with a lower aggregate durability. Even Decimator/Phantom squeaks by, and it has a lot of nasty tricks up its sleeve for making you miss.
Can durability help us understand Dom's weird list from last year's Nationals--three Lambdas and Soontir? Well, it's nearly as durable as BBBBZ, and it brings a high-PS arc-dodger to boot. So that makes sense.
Why does the E-wing struggle? It's about 15% more durable than a B-wing, under MJ's assumptions, with the same attack, a weaker PS, and in at least one important sense a weaker dial--yet it costs 23% more. That may not sound like much of a difference, but if cost were proportional to the durability, the E-wing would cost 25 points instead of 27. (And its durability is also noticeably more variable than the B-wing's, with a standard deviation that's 30% larger.)
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So I won't go on. I'm not trying to claim that differences in durability explain everything about the game, but I'm hoping to make the point that you can actually understand a lot about how the "under the hood" math affects the game by looking at just that one chart--and you can leverage that knowledge to build stronger lists.
Edited by Vorpal Sword