Earlier in the week The Arc did a post on how much past seasons affect its current AFL ratings. There were a few teams that were notably affected by their performances in past seasons, but in general by this stage of the season it’s the current season that primarily matters. This gave me an idea though:
Working out the effect of past seasons in my rankings is pretty easy. My rankings are just based on a team’s past 22 matches, with more recent matches given more weight. Only the current and previous season matter and, unlike The Arc’s ratings and some other rating systems, teams do not regress towards the average between seasons – Round 1 in the current season just carries on from a team’s last game in the previous one.
The weights given to the previous season’s matches go down during the year. After one round into the season they are still up at about 91 per cent of the total, but after Round 5 they are down to about 60 per cent, after Round 9 they are about 36 per cent, and currently – after Round 16 – they are down to just 11 per cent.
Hence, as you can see from the table below, last season’s matches don’t matter much by this stage of the season. No team gains or loses more than about five ranking points from its results in 2016. Essendon, not surprisingly, is held back a bit by its weak, personnel-affected performances last year. Last year’s grand finalists – Sydney and the Western Bulldogs – are held up a bit by their strong performances in last year's finals series.
An interesting case is Sydney, which as The Arc noted were a top team in 2016 but has performed less well in 2017, particularly early in the season. As you can see in the graph below, when the Swans performed badly at the start of the 2017 their ranking points dropped fairly consistently from round to round as last season’s accumulated points fell away.
However, after Round 7 Sydney’s ranking points have been stable, as its improvement in this season has offset its loss of points from 2016. The result is that the Swans are still among the better teams even without last season’s performances holding them up.