Friday, August 7, 2009

AFL 2009 Season - Round 19 Predictions

Predictions for Round 19 of the AFL season follow here. This model is built on a combination of team based data - that is, the ratings for each teams based on previous results - and player based data - so the actual set of player named each week are used as inputs to the model. In effect this means if a star player was replaced at the last minute and replaced by a rookie, then their team's chance of winning is reduced.

As it stands the model is sitting on 105.5 tips for the season (counting the draw as half a tip). At the moment the leading Herald Sun "expert" tipster is on 104 tips, whereas celebrity tipster Shane Warne is on 106, no doubt buoyed in some part to tipping St Kilda every week - so the signs of this model's success look quite promising.

Of course another application for this model is to bet on the outcome of games. An easy way to see if it is worth betting on a team is to multiply the probability of them winning by the odds available, with a result over 100% indicating an advantage over the bookmaker. For example, Essendon are available at odds of $2.60 - so 63% x 2.60 = 163%, so a 63% advantage over the bookmaker.

Once an advantage is found the next question is how much to bet on the team? There are numerous methods, however the method I have used for years is a (not surprisingly) mathematically proven technique called Kelly Criteria. Some useful information can be found about it by reading these pages:

To use the example above, with our 63% advantage over the bookie for Essendon, the amount to bet would be 63% / (2.60 - 1) = 39% of your bank. Of course, this is a large amount of your bank to stake on one game, and most professional punters betting on numerous games at once will scale this figure back quite a bit.

For the record, this model is sitting on a +21% return on investment for the year so far, which is an extremely good outcome in any scenario.

I'll discuss more about the gambling side of these predictions shortly, but for now here are the probabilities for the upcoming round. Interesting that the Western Bulldogs are rated near-certainties, even at odds of $1.05 they are still apparent value.

NOTE - the predictions for the 3 Sunday games have been altered from the original picks to account for the finalised squads

Team 1Team 2VenuePr T1Pr T2
CarltonGeelongMCG37.1% 60.9%
Western BulldogsWest CoastTelstra Dome97.1% 2.7%
HawthornSt KildaAurora Stadium37.3% 60.7%
EssendonBrisbaneMCG63.4% 34.6%
AdelaideCollingwoodAAMI Stadium58.2% 39.7%
North MelbourneMelbourneTelstra Dome82.5% 16.3%
RichmondSydneyMCG56.6% 41.4%
FremantlePort AdelaideSubiaco43.5% 54.4%

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