Wednesday, February 9, 2011

Introducing the AFL Power Rankings - Part Three

OK so far I have introduced the formula for the AFL Power Rankings, which is built around the concept of relative adjusted net margin, where points for versus points against are adjusted for the strength of the opposition and the state where each game is played. I have also talked about what adjustments could be made to net margins, based on the historical results of teams when they travel interstate. Now it is time to calculate us some numbers.

When calculating the numbers, I made one major change to the methodology I had outlined to date. In a previous post, I had proposed using the following formula:

Team Score = 2/3*(Average relative adjusted net margin over past 22 games) + 1/3*(Average relative adjusted net margin over past 5 games)

But I have since replaced it with this formula:

Team Score = Sum over past 22 games of (Weight for game * Relative adjusted net margin for game)

Where weight for game = (1/22) + (1/22) * (11.5 – No. of games ago)/11.5

Essentially the difference is that, in the latter formula, each game has a progressively lower weight the less recent it is. This seemed to me a better weighting pattern than my first formula where, for example, every game from 6 games ago to 22 games ago had the same weight.

Applying this formula gives us the following rankings after the 2010 AFL home-and-away season:

1. Geelong 35.6
2. Collingwood 34.0
3. Western Bulldogs 17.1
4. Hawthorn 16.0
5. St. Kilda 9.6
6. Carlton 5.9
7. Sydney 4.0
8. Adelaide 3.5
9. Melbourne -5.7
10. North Melbourne -7.6
11. Fremantle -9.4
12. Port Adelaide -19.1
13. Brisbane -20.1
14. Essendon -23.5
15. Richmond -24.1
16. West Coast -24.8

Given what we know transpired in the finals, it may seem that Collingwood and St. Kilda are too low. However, while it may be difficult to remember this fact now, Geelong and Collingwood were roughly equal favourites for the premiership heading into the finals series, with Geelong coming off thumping wins against the Bulldogs and Carlton heading into September, and Collingwood squeaking past Adelaide and losing (albeit narrowly) to Hawthorn. Meanwhile, St. Kilda had a so-so second half of the season, only winning three and drawing one of its last seven games.

The rankings of Hawthorn and Fremantle may also be controversial, given that the Dockers polished off the Hawks in their first final. But leading up to that game Hawthorn were displaying much better form than Fremantle, with the Hawks making up for their poor start to the season, and the Dockers squandering their great start. If you take away Fremantle’s 116-point thrashing in Hobart, where it fielded a second-string side, it would move up to ninth in the rankings, which is perhaps a fairer reflection of its form.

If we factor the 2010 finals in, we come up with the following rankings:

1. Collingwood 39.8
2. Geelong 30.6
3. St. Kilda 12.9
4. Hawthorn 12.0
5. Western Bulldogs 8.2
6. Carlton 4.2
7. Sydney 3.4
8. Adelaide 1.6
9. Melbourne -7.1
10. North Melbourne -10.0
11. Fremantle -11.1
12. Port Adelaide -21.0
13. Brisbane -21.7
14. Essendon -25.4
15. Richmond -25.9
16. West Coast -26.3

Collingwood has now pulled clear of Geelong, and St.Kilda has moved up to third, reflecting the good form of both of those teams during September. In terms of its score, Hawthorn has been punished for its first-up loss in Fremantle, although it remains in fourth position overall. (The scores of non-finals teams also change as all of the relative margins change, but only by small amounts.)

I’m reasonably happy with the post-finals rankings, so I think this formula will stick. That being the case, the Depot will bring you the updated AFL Power Rankings every week throughout the season (though with considerably less discussion). Stay tuned.

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