Random walks and momentum shifts in the 2018 Super Netball Grand Final

Introduction

The Sunshine Coast Lightning defeated West Coast Fever in the 2018 Super Netball Grand Final.

Prior to the game, I had identified some performance bandwidth profiles for both teams. Post-game, the performance profiles of both teams were:

Sunshine Coast

The team’s actual performance delivered an exceptional third quarter and moved the team into a game-winning position.

West Coast

The scoring sequence in the game was:

Random Walks

In 2006, Martin Lames explored dynamic interactions in sport games. His discussion included an examination of handball possession in terms of interlaced random walks and the momentary strength of competing teams.

Martin conjectures in his paper:

There is evidence for the hypothesis that a team’s scoring rate is independent from the one of other team, but we see also phases with a seemingly strong dependence. Moreover, sometimes the momentary scoring probabilities seem to be negatively correlated (my team is good when the other is bad and vice versa), but sometimes there is a positive relationship (my team performs well when the other does so).

There have been subsequent discussions of random walk in the literature. See, for example: Alan Gabel and Sidney Redner (2012); Leto Peel and Aaron Clauset (2015); Dilan Kiley et al (2016); and Jaime Prieto, Miguel-Angel Gomez and Jaime Sampaio (2016).

In their discussion, Alan and Sidney noted:

There are three factors that determine which team scores. First, the better team has a greater intrinsic chance of scoring. The second factor is the anti- persistence of successive scoring events that arises from the change of possession after a score. The last is the linear restoring force, in which the scoring probability of a team decreases as its lead increases (and vice versa for a team in deficit).

Leto and Aaron propose:

Anti-restoration or momentum occurs when the leading team has a higher chance of scoring again.

Momentum is the reverse of restoration.

Dilan and his colleagues note:

Each game generates a probabilistic, rule-based story, and the stories of games provide a range of motifs which map onto narratives found across the human experience: dominant, one-sided performances; back-and-forth struggles; underdog upsets; and improbable comebacks.

The Final

I used secondary day from Champion Data’s record of the Final. I though I would look for random walks and momentum changes in the data.

I used RStudio, ggplot2, and ggrepel to visualise the data. My record of the game tracks the Sunshine Coast’s score difference performance. Green shaded areas indicate Sunshine Coast lead. The second quarter is in purple to indicate West Coast’s lead throughout that quarter.

The data show eight excellent examples of the tendency of a random walk to move to a central location. Each of them exists at a new equilibrium in the game.

I have included some time-in-game labels to indicate my perception of a momentum shift. The ability for a team to create these episodes (and respond to them when opponents are driving the game) resonates powerfully with some of the discussions about temporal (T) patterns and their critical interval relationships initiated by Magnus Magnusson.

It would be fascinating to learn how coaches from both teams addressed these shifts in the messages they shared with their players. It would be interesting to learn what was said at half time too. The half time break straddled a seven-goal run from the Sunshine Coast.

Discussion

I have really enjoyed this season’s Super Netball competition. The final was closely contested. I was particularly interested in the pivot in the game that occurred in the third quarter. The Sunshine Coast produced their best third quarter of their entire season. Their previous highest score in the third quarter was 18. West Coast had experience of teams lifting in the third quarter in recent games. Their opponents in weeks 13 and 14 of the regular season had both scored 20 goals.

My pre-game priors suggested that West Coast would win the Final by 3 goals. At 13 minutes and 05 seconds in the second quarter West Coast led by 7 goals. What happened thereafter provides a case study of the interlacing of random walks in the context of momentum shifts … for an away team.

Photo Credits

Super Netball (Sue Gaudron Q & A)

Lightning (Twitter)

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