2018 - Round 22 : Overs/Unders Update

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If, as it seems, the mean total score for an AFL game in 2018 is 165 to 170 points, last weekend was a textbook example of regression to the mean after the relative scorefest that was Round 20.

Round 21 produced about 169 points per game, which is what the consensus average is for this week as well.

If the bookmakers are correct, three or four games will breach the 170 threshold this week; if the MoS twins are correct, four to six games will.

There's mostly agreement again this week about which will most-likely be the high- and low-scoring games and teams.

(Note that Centrebet is no more as of this week, having morphed to become BetEasy. For the remainder of this season I'll continue to use the term Centrebet for convenience.)

MOST LIKELY HIGH-SCORING GAME

  • MoSSBODS: Gold Coast v Brisbane Lions, and Adelaide v North Melbourne (183)
  • MoSHBODS, TAB & Centrebet: Adelaide v North Melbourne (183 to 188.5)

MOST LIKELY LOW-SCORING GAME

  • All: Collingwood v Port Adelaide (147 to 150.5)

MOST LIKELY HIGH-SCORING TEAM

  • All: Geelong (104 to 111)

MOST LIKELY LOW-SCORING TEAM

  • All: Fremantle (52 to 61)

MoSHBODS' performance was excellent for predicting the high- and low-scoring teams and games last weekend, with every one of its highest probability selections proving the correct choice. Specifically, it attached:

  • 19.6% probability to the highest-scoring team, Richmond (against a naive forecast of 5.6% since 18 teams were playing)
  • 27.0% probability to the lowest-scoring team, Gold Coast (against a naive forecast of 5.6%)
  • 16.8% probability to the highest-scoring game, Essendon v St Kilda (against a naive forecast of 11.1% since 9 games were played)
  • 23.0% probability to the lowest-scoring game, Port Adelaide v West Coast (against a naive forecast of 11.1% since 9 games were played)

Overall this season, MoSHBODS has predicted better than a naive forecaster in selecting the high- and low-scoring teams, almost exactly as well as a naive forecaster in selecting the high-scoring games, and worse than a naive forecaster in selecting the low-scoring games.

By taking the bookmakers' implied estimates of expected team scoring in each game and then using these expectations to simulate the games in each round, in exactly the same way as estimates are created from MoSHBODS' scoring expectations, I'm able to assess their performance against a naive forecaster as well. Doing this suggests that the TAB and Centrebet bookmakers have both performed better than a naive forecaster (and MoSHBODS) in all four markets so far this season, with Centrebet outperforming the TAB in terms of log probability score in each, though only very slightly so in the low- and high-scoring teams markets.

This week, MoSHBODS has

  • seven teams as more likely than 1 in 18 to be the highest-scoring: Adelaide, Geelong, Brisbane Lions, GWS, Western Bulldogs, and Hawthorn.
  • seven teams as more likely than 1 in 18 to be the lowest-scoring: Fremantle, Carlton, Port Adelaide, Sydney, Carlton, Essendon, St Kilda, and North Melbourne.
  • four games as more likely than 1 in 9 to be the highest-scoring: Crows v Roos, Suns v Lions, Eagles v Dees, and Tigers v Dons.
  • three games as more likely than 1 in 9 to be the lowest-scoring: Pies v Power, Giants v Swans, and Blues v Dogs.

WAGERS

Investors have just four wagers this week comprising one unders and three overs bets on overlays ranging from about 1 to 2.5 goals. 

The relevant weather forecasts are as follows:

  • Richmond v Essendon (Friday, MCG - overs bet): Partly cloudy. 30% chance of rain.
  • Geelong v Fremantle (Saturday, Kardinia Park - overs bet): A few showers. 70% chance of rain.
  • Carlton v Western Bulldogs (Sunday, Docklands - unders bet): A few showers with a high chance of roof.
  • West Coast v Melbourne (Sunday, Perth Stadium - overs bet): Sunny. 20% chance of rain.

So, really only the Geelong v Fremantle game looks a possible candidate for adversely adverse weather. Let's hope the 2.5 goal overlay is sufficient there or the rain decides to go watch another game. 

PERFORMANCE TO DATE

MoSSBODS finished on the right side of TAB and Centrebet totals in six games last week, while MoSHBODS managed only three against both bookmakers.

That left MoSSBODS' with a season-long 49% record against the TAB and against Centrebet, and left MoSHBODS with a 47% record against the TAB and a 48% record against Centrebet. 

In games where it wagered, MoSSBODS' landed 1 from 2 overs bets and 1 from 1 unders bets with the TAB, as well as 1 from 1 unders bets with Centrebet. That lifted its overall strike rate on overs bets across the season to 34% and on unders bets to 56%.

On the mean absolute error (MAE) metric, the TAB registered the lowest MAEs this week on home team scores and on away team scores, and tied with Centrebet for low MAE on game margins. MoSSBODS registered the lowest MAE for game totals.

That left Centrebet still leading on the season-long view for home team scores (MAE of 17.5), game margins (26.0), and game totals (22.2). The TAB still leads, narrowly, on away team scores (17.3).