Which changes move the meta
Every line in the notes looks the same size. Read them by how many boards they touch, and check how the last patch's changes landed.
Checked for patch 18.4. The numbers come from the live Set 18 data and follow every patch.
The idea
The notes print every change at the same size, one line each. Their effect on your games is nothing like the same. A change matters as far as the boards it touches, and every lobby you play holds 8 of them.
Start with play. A nerf to a unit on a big share of boards changes a lot of games at once, yours included even when you never play it, because you meet it across the lobby. A buff to a unit almost nobody plays has to lift it a long way before it shows up in your games at all.
Then ask how far the change reaches. A trait change reaches every unit with the trait and every comp that runs it. An item change reaches every unit that holds the item. A change to a mechanic reaches every board. One champion's numbers reach the comps built around that champion, and cost decides when: a 1-cost fills 75% of every shop slot at level 3, while a 5-cost fills 3% at level 8 and 15% at level 9, so a 5-cost change only lands in games that get that far.
Last, a change on paper is not a change in games. The whole patch moves at once: a buffed champion can do worse if its comp lost a trait or an item, a nerfed one can hold if its comp gained elsewhere, and a comp nobody touched can rise or fall because the comps around it did. The notes tell you where to look; a few days of games tell you what happened.
See it live
How patch 18.4's changes landed: every champion it changed, and the comps that moved most, against patch 18.3.
Champions patch 18.4 changed, patch 18.3 against 18.4
Share of boards that played it and its average placement. Platinum and above, every server, the last three days of each patch.
Kennen Nerfed25.3% → 21.4% of boards3.89 to 3.95 average
Ivern Adjusted21.2% → 18.7% of boards3.84 to 3.93 average
Sivir Buffed18.5% → 8.8% of boards4.45 to 4.66 average
Ashe Adjusted17.2% → 9.6% of boards3.97 to 4.17 average
Rakan Buffed15.9% → 11.3% of boards4.80 to 4.92 average
Alistar Buffed15.3% → 17.2% of boards4.53 to 4.50 average
Veigar Nerfed15.2% → 14.6% of boards4.33 to 4.38 average
Ezreal Buffed14.9% → 18.2% of boards4.35 to 4.30 average
Nidalee Adjusted14.6% → 10.7% of boards4.30 to 4.36 average
Hecarim Buffed14.4% → 20.0% of boards4.73 to 4.61 average
Comps that moved most from 18.3 to 18.4
Average placement, each comp found by its board on both patches.
Vanguard Morgana5.29 to 4.11
Lunar buffed, Coven nerfed.
Blossom Karma5.06 to 4.61
Kayle, Blossom and Spirit Visage buffed, Juggernaut nerfed.
Riftbeast Pebbles4.58 to 4.14
Riftbeast, Murkwolf and Spirit Visage buffed, Brambleback adjusted.
Juggernaut Zyra Amumu4.31 to 4.70
Sivir and Spirit Visage buffed, Juggernaut nerfed, Ashe adjusted.
Fae Tristana4.70 to 5.03
Rengar, Tristana, Rakan, Sivir and Spirit Visage buffed, Rammus, Fae and Juggernaut nerfed.
Juggernaut Sivir4.34 to 4.67
Sivir, Rakan and Spirit Visage buffed, Juggernaut and Kennen nerfed, Ashe and Ivern adjusted.
The rule of thumb
Read the notes from the most played thing down: a change matters as far as the boards it touches.
Check yourself
Patch 18.4 changed Kennen, on 25% of boards the patch before, and Gromp, on 2%. Which change will you meet in more of your games?
Pick an answer.