You can isolate winrate deltas for specific changes? That's very cool. Can you share a little more about that? Is it a matter of referencing similar changes in the past, theorycrafter-style math, etc.?
2% is surprising if it is coming from the cost changes alone. I would expect huge differences like that to come from some sort of first clear breakpoint, but Karthus would have just -1 mana per Q once he levels it again at rank 3. I think he has remained pretty hard-bound to PoM secondary and Liandry's; am I missing an important factor here?
Tends to be using similar or the same changes in the past to ballpark. Like we had previously done this exact Karthus Q ap ratio the other way so we have a decent sense of that change. We also have good senses of base stats and stuff like simple damage changes.
Karthus was on the high end here. I looked into it and he had a total of like a 5-6% delta that patch. A lot of it for Karthus was he was actually under the bar for being able to clear the jungle midgame so he's absolutely on the high end, it moved him from like running out of mana to being ok with mana itemization