STATS DROP: Round 18, 2023

Homeground advantage, Eels and Warriors Elo, Raiders Pythag, one-win QCup teams, Tony Stewart, Form Elo, Pythagorean expectation, 2nd order wins, WARG and TPR for NRL and Queensland Cup

Jul 06, 2023

Welcome to Stats Drop, a weekly inundation of rugby league numbers.

When you really think about it, homeground advantage is weird. There’s plenty of data to prove that it exists. Covid showed that it’s at least partly the crowd. Travel for the away team also seems to be a component. The precise ratio of these factors and the impact of others, like familiarity with the ground, is not always clear. Whether the advantage is constant through time, or variable, has probably not been tested. From year-to-year there’s a large amount of inexplicable noise1.

While over the long run, playing at home is worth about a try all other things being equal, trying to determine which teams have a greater homeground advantage than others is difficult.

The noise is made louder by the performance of the individual teams in question. A very good team wins both home and away games, largely in equal measure, which tells you nothing about their home ground advantage. A very bad team loses both home and away games, which doesn’t offer many clues as to whether their home results would be worse without the in-built advantage. Everyone in between wins and loses in differing ratios, of which homeground advantage has some, but not all, explanatory power.

For example, we would expect that North Queensland enjoys a significant homeground advantage due to a decent attendances, a high level of parochiality and Townsville is a long way from anywhere else. But if the Cowboys are bad (or good!) for many years, then how much of their results at home can be attributed to the home ground advantage and how much to the team itself?

To cut through some of the noise caused by performance, I looked at the long term ratio of wins collected at home as a proportion of a team’s total wins.2

There’s something here. We would expect the Warriors and Hunters to have the toughest road trips in their respective leagues but also probably struggle most with the travel demands for their away games.

But there are oddities. Mackay and Townsville are much lower than we’d expect given their relative remoteness, although they are at different ends of the performance spectrum. Manly and St George Illawarra are much higher than we’d expect, given their location in the eastern half of Sydney and most of their rivals having less than two hours on the road to reach them.

It’s also strange to contrast the Broncos, who have exactly one home ground and have occupied that ground since 2003 and it’s so popular, other teams take their home games there, and the Tigers, who have multiple home grounds, both in Sydney but also in regional New South Wales and, sometimes, New Zealand. Despite all these differing factors, their respective ratios are pretty much identical.

Perhaps this is a mystery that will never be fully solved.

Trivia

People aren’t high enough on the Eels and are too high on the Warriors. Elo don’t lie3.
Canberra are currently on pace to finish with 3.3 wins over their Pythagorean expectation. Only the 2003 Panthers (+3.5), 2006 Storm (+3.6) and 2007 Cowboys (+4.4) had bigger outperformances. Two of them topped the ladder and one won a premiership. There are seven teams that underperformed by a greater margin, the best being the 2002 Eels and the most recent, the 2008 Cowboys. We haven’t seen a discrepancy like this in 15 years.
Ipswich are currently winless through 15 games. One of the footnotes on Monday covered the six winless teams in Queensland Cup history but the Jets are projected by Elo to win one (1) game. There have been 8 one-win teams in Queensland Cup: 96 and 14 Sunshine Coast (1-14 and 1-23 respectively), 97 Ipswich (1-16-1), 99 Souths (1-21) and 05, 15, 19 and 21 Central (1-17-2, 1-22, 1-21-1 and 1-14-2 respectively).
Tony Stewart killed a guy.

NRL Stats Drop

Scroll to the end for the explanatory notes or visit How It All Works.

Queensland Cup Stats Drop

Explanatory notes

Elo Ratings

Elo ratings are a way of quantitatively assessing teams, developing predictions for the outcomes of games and then re-rating teams based on their performance, home ground advantage and the strength of their opposition. Form Elo ratings are optimised for head-to-head tipping and tend to reflect the relative strengths of each team at that particular point in time, although there are many factors that affect a team’s rating.

Pythagorean Expectation and Winning Percentage Comparison

The black dots on the winning percentage comparison are each team’s actual win-loss record to this point in the season. The coloured dots represent what the stats say about the team’s underlying performance, i.e. how many games they should be winning. Wins and losses are binary and can be prone to good and bad luck in a way that other stats that correlate to wins are not, so we have other metrics to help see through the noise to good teams, rather than just good results. While each metric has strengths and weaknesses, it helps paint a more complete picture of team performance.

Pythagorean expectation (gold) relies on points scored and conceded by estimating a team’s number of wins based on their for and against. Where there is a deviation between a team’s actual record and their Pythagorean expectation, we can ascribe that to good fortune, when a team wins more than they are expected to, or bad fortune, when a team wins less than they are expected to. 2nd order wins (silver) relies on metres and breaks gained and conceded. Elo ratings (maroon) rely on the margin of victory and strength of opponent.

Dots should tend to gravitate towards each other. If a team’s dots are close together, that means their actual results are closely in line with their underlying metrics and represents a “true” or “fair” depiction of how good the team is. If a team’s coloured dots are clustered away from their actual record, then we should expect the actual and the coloured cluster to move towards each other over time.

If the black dot is well above gold, that team is suffering from good fortune and may mean regress to more typical luck in the future (vice versa also holds). The silver dots will tend to hover around .500, so if gold is between silver and .500, the team could have an efficiency issue. On the balance, I would expect more often the actual percentage will move towards the cluster but the opposite is also possible.

Player Leaderboards

Production the amount of valuable work done by a team as measured in counting statistics that correlate with winning. These statistics are converted to a single unit called Taylors. Taylor Player Ratings (TPR) are a rate metric that compares an individual player’s production, time-adjusted, to that of the average player at their position, with a rating of .100 being average (minimum 5 games played). Wins Above Reserve Grade (WARG) is a volume metric that converts player’s production over a nominal replacement level into an equivalent number of wins they contribute to their team.

Team Strengths

A chart that tabulates each team’s WARG, both as a team total and broken down into platoons by listed position, to identify each team’s relative strengths and weaknesses.

The WARG is the sum produced at the position, not by specific players, e.g. if Kalyn Ponga produces a total of 1.5 WARG comprising 1.0 WARG while named at fullback and 0.5 WARG named at five-eighth, then the WARG produced while named at fullback will be added to the 1-5 platoon and the WARG created while named in the halves will be added to the 6+7+9 platoon.

The top team in each category is rated 100, the bottom team is rated 0 and every team in between is scaled accordingly.

Team Efficiencies

This table compares the SCWP produced and conceded by each team (a product of their metres and breaks gained and conceded) against the actual points the team scores and concedes to measure which teams are most efficiently taking advantage of their opportunities. A lack of efficiency here could be the result of bad luck and poor execution - sometimes you have to watch the games.

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2020 and 2021 in the NRL is somewhat easier to explicate.

Note that Auckland and New Zealand are counted as one, as are the Northern Eagles and Manly (North Sydney is standalone). The predecessor clubs for the mergers, Balmain/Wests, St George/Illawarra, Southern Suburbs/Logan City, are all treated as separate from their joint ventures.

It does mislead but in this case, it’s confirming my priors.