We hope you enjoyed our previous write-up in Round 2 where we found a winner in Adelaide who covered the spread in a dominant win away against Collingwood. This week we thought it would be interesting to look at a game where the final team announcements had a significant impact on our projected line.

The Western Bulldogs have been the surprise packet of the season to date, notching very strong wins against West Coast, Richmond, Adelaide and most recently Sydney at the SCG no less. After a tumultuous off season where they lost some of their best and most experienced players to other clubs and injuries (Cooney, Griffen and Liberatore), we, like many, were quite low on the Dog’s prospects at the start of the year. Credit must be given to the young playing group and their new coach however, as they are playing an exciting brand of footy that has the AFL world talking.

St Kilda have been up and down so far this season, and having their two most experienced players in Riewoldt and Montagna on the sidelines the past few weeks has not helped their cause. They are in a rebuild phase looking to develop their next generation, and there have definitely been some promising signs for Saints fans. They had an impressive win in round 2 this year, albeit against a club in turmoil in the Gold Coast, and last week they pushed Essendon all the way in a Sunday thriller.

This weekend St Kilda welcome their two sidelined veterans back into their line-up, while the Bulldogs exciting young midfielder Lin Jong and seasoned veteran Matthew Boyd will be forced to miss due to injury. These line-up changes have a significant impact on the player ratings of these two squads, which is illustrated in the graph below.

SaintsDogs Graph

You can see that from a raw player ratings perspective, the Bulldogs ratings plummet and St Kilda’s aggressively rise, basically to be the reverse of what they were in Round 5. It is worth noting that the Bulldogs rating is right on the rating of the team that was destroyed by Hawthorn in Round 3. Coincidentally, our biggest bet of the year to date was against the Bulldogs that day on Hawthorn (-42.5).

As you can guess, these personnel changes have a significant impact on our pricing of the game, as the player ratings side of things is one part of a two component pricing model we use. The following should provide some colour on how the game is priced once final teams are announced:

Initially, we run our statistical team based model, which crunches team based data from a selection of past matches (mainly from 2014 and the first 5 rounds of 2015) to come up with a team based line projection. We run this model multiple times, tweaking how much weighting each historic game gets. We focus on weighting previous matches between the two teams, and games where teams have a similar player rating to their named line-up for the weekend (if possible). Our team based model’s projection for this game was Western Bulldogs by 22 points.

Next, we run the player based component of the model. We take the weighted average aggregate player rating of each team, based on the weightings we gave to each of their historic games that were inputs into the model. We then look at the relative strength of each team for the coming weekend compared to their weighted average strength input into the model, and make a mathematical line adjustment based on any differences.

Western Bulldogs player rating adjustment: As highlighted by the graph above, the Bulldogs team is well below strength, and therefore the weighted average of their previous games’ player ratings input into the model are higher than their aggregate player rating heading into this weekend. Therefore they attract a negative adjustment, moving our line back towards St Kilda.

St Kilda player rating adjustment: As highlighted by the graph above, the Saints team is the strongest they have been to date in 2015, and therefore the weighted average of their previous games’ player ratings input into the model are lower than their aggregate player rating heading into this weekend. Therefore they attract a positive adjustment, moving our line further towards the Saints.

Final Line = (Team based model projection) + (Home team adjustment) – (Away team adjustment) = Western Bulldogs -8.7

Despite St Kilda having a higher aggregate player rating this weekend, AFL is still a team game and we still price the Western Bulldogs as favourites. Our model is about relative strength, and the adjustments are calculated based on the magnitude of the difference in player ratings to the weighted average input into the model, rather than just the raw difference between two teams, as that would ignore team based factors such as coaching and team play.

Prediction: Western Bulldogs by 9 points.

Bet 1: St Kilda +18.5 @ $1.925 (Pinnacle)
Rated price = $1.65, expected ROI = 16.8%

Bet 2: St Kilda h2h @ $3.05 (Pinnacle)
Rated price = $2.47, expected ROI = 23.5%

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