Monday, February 7, 2011

Introducing the AFL Power Rankings - Part Two

In my previous post on a proposed ‘Power Rankings’ system for AFL teams, I talked about the concept of adjusted net margin, where the net margin for a team (points for – points against) would be adjusted by the relative advantage/disadvantage from playing home/away. I said there that the adjustments would need to be worked out; in this part I offer some opinions on what those adjustments could be.

To answer this, I calculated the average losing margins of ‘away’ teams at the seven main home grounds of AFL teams: AAMI Stadium in Adelaide, the GABBA in Brisbane, Skilled Stadium in Geelong, the MCG and Etihad Stadium in Melbourne, Subiaco Oval in Perth, and the SCG in Sydney. I did this from 1987 onwards, when West Coast (and the Brisbane Bears) joined the league, but did not bother including the Bears or Fitzroy in my calculations. For the two Melbourne grounds, I assumed that those grounds were ‘home’ grounds for all of the Victorian teams. There were some differences in the Victorian teams' performance between the two grounds, but I think that has more to do with how successful each team has been since Etihad Stadium opened in 2000.

The average losing margins of ‘away’ teams at each ground were as follows:

Skilled Stadium – 22.6
AAMI Stadium – 18.7
GABBA – 18.6
Subiaco – 12.2
MCG – 8.0
Etihad – 7.0
SCG – 6.6

From these figures, it seems that Geelong, the Adelaide teams, and Brisbane have the highest home ground advantages, but one thing that needs to be remembered is all those teams have been fairly successful over the past couple of decades. For example, Geelong’s average net margin since 1987 is itself over two goals. If you look at those teams’ overall average net margins – that is, roughly how well those teams would be expected to do at a ‘neutral’ venue – then I reckon their home ground advantage is worth about two goals in each case. I also think the home ground advantage at Subiaco is worth about two goals, once you account for West Coast’s and Fremantle’s overall performances.

The SCG result is a little surprising, even accounting for the fact that it includes some games where two ‘away’ teams played (for example, when North Melbourne and the Bulldogs played ‘home’ games there). My first thought was to say that the MCG, Etihad and the SCG all have a home ground advantage of about nine points against all interstate teams. But looking closer at the figures, I noticed that Sydney tends to perform better in Melbourne than other interstate teams, and Victorian teams tend to perform better in Sydney than other interstate teams. Therefore, I reckon a better approach is to say that playing at the SCG is worth one goal advantage to Sydney against Victorian teams, and two goals against all others, and that playing at the MCG or Etihad is worth one goal to Victorian teams against Sydney, and two goals against all other interstate teams. (If you hadn’t already worked it out, I’m setting the advantage of playing in Melbourne against another Victorian team as zero.)

Those results seem fairly reasonable to me, so now I have pretty much all I need to go calculate some ranking points.

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