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Ah yes, the Cassiopeia "nerfs" that essentially buffed her early laning against every AP match-up.
As for Volibear's pick rate sitting at 1.44%, that must be a mistake. He's a freakin' raid boss atm and I see him get picked every other game at D4 MMR both for top lane and jungle.
The change was meant as Top/Bot nerfs. They were definitely neutral-to-positive for mid lane in most matchups.
The Brand "buff" changed close to nothing for Brand support and the winrate still went down by 0.89%. It shows that there is not enough data to draw conclusions when the expected change is in 0.X percentages. The only safe conclusion here is that Voli's buff was meaningful (or people just got better playing him).
Almost always, win rate changes are "Here was the average win rate on the patch before. Here's the average win rate on the new one." It almost always skips over the win rate growth gained from familiarity over the last two weeks. So yeah, without doing some decent crunch to solve that issue, tracking patch-over-patch winrate growth is pretty useless for tracking buffs/nerfs.
Cassiopeia already excelled at mid lane. She could stand being nerfed for Top/Bot and not get any compensation for her mid laning. That's my point.
To be fair +0.5% within a week on public data is within the margin of error for "just as powerful." Like we saw with Brand losing almost 1% in support despite being objectively more powerful.