Pies v Saints: An Initial Prediction

During the week I'm sure I'll have a number of attempts at predicting the result of the Grand Final - after all, the more predictions you make about the same event, the better your chances of generating at least one that's remembered for its accuracy, long after the remainder have faded from memory.

In this brief blog the entrails I'll be contemplating come from a review of the relationship between Grand Finalists' MARS Ratings and the eventual result for each of the 10 most recent Grand Finals.

Firstly, here's the data:

In seven of the last 10 Grand Finals the team with the higher MARS Rating has prevailed. You can glean this from the fact that the rightmost column contains only three negative values indicating that the team with the higher MARS Rating scored fewer points in the Grand Final than the team with the lower MARS Rating.

What this table also reveals is that:

• Collingwood are the highest-rated Grand Finalist since Geelong in 2007 (and we all remember how that Grand Final turned out)
• St Kilda are the lowest-rated Grand Finalist since Port Adelaide in 2007 (refer previous parenthetic comment)
• Only one of the three 'upset' victories from the last decade, where upset is defined based on MARS Ratings, was associated with a higher MARS Rating differential. This was the Hawks' victory over Geelong in 2008 when the Hawks' MARS Rating was almost 29 points less than the Cats'

From the raw data alone it's difficult to determine if there's much of a relationship between the Grand Finalists' MARS Ratings and their eventual result. Much better to use a chart:

The dots each represent a single Grand Final and the line is the best fitting linear relationship between the difference in MARS Ratings and the eventual Grand Final score difference. As well as showing the line, I've also included the equation that describes it, which tells us that the best linear predictor of the Grand Final margin is that the team with the higher MARS Rating will win by a margin equal to about 1.06 times the difference in the teams' MARS Ratings less a bit under 1 point.

For this year's Grand Final that suggests that Collingwood will win by 1.062 x 26.1 - 0.952, which is just under 27 points. (I've included this in gray in the table above.)

One measure of the predictive power of the equation I've used here is the proportion of variability in Grand Final margins that it's explained historically. The R-squared of 0.172 tells us that this proportion is about 17%, which is comforting without being compelling.

We can also use a model fitted to the last 10 Grand Finals to create what are called confidence intervals for the final result. For example, we can say that there's a 50% chance that the result of the Grand Final will be in the range spanning a 5-point loss for the Pies to a 59-point win, which demonstrates just how difficult it is to create precise predictions when you've only 10 data points to play with.

Visualising AFL Grand Final History

I'm getting in early with the Grand Final postings.

The diagram below summarises the results of all 111 Grand Finals in history, excluding the drawn Grand Finals of 1948 and 1977, and encodes information in the following ways:

• Each circle represents a team. Teams can appear once or twice (or not at all) - as a red circle as Grand Final losers and as a green circle as Grand Final winners.
• Circle size if proportional to frequency. So, for example, a big red circle, such as Collingwood's denotes a team that has lost a lot of Grand Finals.
• Arrows join Grand Finalists and emanate from the winning team and terminate at the losing team. The wider the arrow, the more common the result.

No information is encoded in the fact that some lines are solid and some are dashed. I've just done that in an attempt to improve legibility. (You can get a PDF of this diagram here, which should be a little easier to read.)

I've chosen not to amalgamate the records of Fitzroy and the Lions, Sydney and South Melbourne, or Footscray and the Dogs (though this last decision, I'll admit, is harder to detect). I have though amalgamated the records of North Melbourne and the Roos since, to my mind, the difference there is one of name only.

The diagram rewards scrutiny. I'll just leave you with a few things that stood out for me:

• Seventeen different teams have been Grand Final winners; sixteen have been Grand Final losers
• Wins have been slightly more equitably shared around than losses: eight teams have pea-sized or larger green circles (Carlton, Collingwood, Essendon, Hawthorn, Melbourne, Richmond, Geelong and Fitzroy), six have red circles of similar magnitude (Collingwood, South Melbourne, Richmond, Carlton, Geelong and Essendon).
• I recognise that my vegetable-based metric is inherently imprecise and dependent on where you buy your produce and whether it's fresh or frozen, but I feel that my point still stands.
• You can almost feel the pain radiating from those red circles for the Pies, Dons and Blues. Pies fans don't even have the salve of a green circle of anything approaching compensatory magnitude.
• Many results are once-only results, with the notable exceptions being Richmond's dominance over the Blues, the Pies' over Richmond, and the Blues over the Pies (who knew - football Grand Final results are intransitive?), as well as Melbourne's over the Dons and the Pies.

As I write this, the Saints v Dogs game has yet to be played, so we don't know who'll face Collingwood in the Grand Final.

If it turns out to be a Pies v Dogs Grand Final then we'll have nothing to go on, since these two teams have not previously met in a Grand Final, not even if we allow Footscray to stand-in for the Dogs.

A Pies v Saints Grand Final is only slightly less unprecedented. They've met once before in a Grand Final when the Saints were victorious by one point in 1966.