(This piece originally appeared in The Guardian newspaper as https://www.theguardian.com/sport/datablog/2017/jun/15/matter-of-stats-afl-datablog-offence-defence)
There are a lot of sportswriters and sports fans who are adamant that it’s impossible to compare sporting teams across seasons and eras. Taken literally, that’s a truism, because every sport evolves, and what worked for a great team in, say the 1980s, might not work – or even be within the rules of the game – today.
Still, if asked to identify some of the best AFL teams of recent years, most would almost certainly include the 2000 Essendon, 2011 Geelong, and 2012 Hawthorn teams. We all have, if nothing else, some intuitive sense of relative team abilities across time.
As imperfect as it is, one way of quantifying a team’s relative ability is to apply mathematics to the results it achieves, adjusting for the quality of the teams they faced. Adjustment for opponent quality is important because, were we to use just raw results, a 90-point thrashing of a struggling team would be treated no differently in our assessment of a team’s ability than a similar result against a talented opponent.
This notion of continuously rating individuals or teams using results adjusted for opposition quality has a long history and one version of the method can be traced back to Arpad Elo, who came up with it as a way of rating chess players as they defeated, drew or lost to other players of sometimes widely differing abilities. It’s still used for that purpose today.
In sports like football, Elo-style rating systems can be expanded to provide not just a single rating for a team, but a separate rating for its offensive and defensive abilities, the former based on the team’s record of scoring points relative to the quality of the defences it has faced, and the latter on its record of preventing points being scored relative to the quality of the offences it has faced.
If we do this for the AFL we can quantify the offensive and defensive abilities of teams within and across seasons using a common currency: points.
There are many ways to do this, and a number of websites offer their own versions, but the methodology we’ll use here has the following key characteristics:
- Teams start with offensive and defensive ratings of zero and this is therefore the rating of an average team in every era
- Ratings are adjusted each week on the basis of offensive and defensive performance relative to expectation
- Final scores are adjusted to account for the fact that on-the-day accuracy – the proportion of scoring shots registered as goals – can be shown empirically to have a large random component
- Every team has a “venue performance value” for every ground, which measures how much better or worse, on average, that team has historically performed at that venue. These values tend to be positive for teams’ home grounds and negative for their away grounds
- Teams carry across a proportion of their end-of-season rating into the next season, which reflects the general stability in lists and playing styles from one season to the next
- Adjustments made to team ratings on the basis of results relative to expectations tend to be larger in the early parts of the season to allow the ratings to more rapidly adjust to any changes in personnel, style or ability
- The numerical value of the ratings has a direct interpretation. For example, a team with a +10 offensive rating would be expected to score 10 more points than an average team when facing an average opponent at a neutral venue. Similarly, a team with a -10 defensive rating would be expected to concede 10 more points than an average team when facing an average opponent at a neutral venue.
- To obtain a combined rating for a team we can simply add its offensive and defensive rating
(For more details see this blog post on the MoSHBODS Team Rating System)
End of Home and Away Season Ratings in the Modern Era
Applying this methodology generates the data in the chart below, which records the offensive and defensive ratings of every team from seasons 2000 to 2016 as at the end of their respective home and away season. Teams that ultimately won the Flag are signified by dots coloured red, and those that finished as Runner Up as dots coloured orange. The grey dots are the other teams from each season – those that missed the Grand Final.
We see that teams lie mostly in the bottom-left and top-right quadrants, which tells us that teams from the modern era that have been above-average offensively have also tended to be above-average defensively, and conversely that below-average offensive teams have tended to be below-average defensively as well.
The level of association between teams’ offensive and defensive ratings can be measured using something called a correlation coefficient, which takes on values between -1 and +1. Negative values imply a negative association – say if strong offensive teams tended to be weak defensively and vice versa – while positive values imply a positive association, such as we see in the chart.
The correlation coefficients for team ratings in the current and previous eras appears in the table at right. We see that the degree of association between team offensive and defensive ratings has been at historically high levels in the modern era. In fact, it’s not been as high as this since the earliest days of the VFL.
In other words, teams offensive and defensive ratings have tended to be more similar than they have been different in the modern era.
By way of comparison, here’s the picture for the 1980 to 1999 era in which the weaker relationship between teams’ offensive and defensive ratings is apparent.
Note that the increase in correlation between teams’ offensive and defensive abilities in the modern era has not come with much of a reduction in the spread of team abilities. If we ignore the teams that are in the lowest and highest 5% on offensive and defensive abilities, the range of offensive ratings in the modern era span about 31 points and defensive ratings span about 34 points. For the 1980-1999 era the equivalent ranges are both about 2 points larger.
One plausible hypothesis for the cause of the closer association between the offensive and defensive abilities of modern teams would be that coaching and training methods have improved and served to reduce the level of independent variability in the two skill sets.
The charts for both eras have one thing in common, however: the congregation of Grand Finalists – the orange and red dots – in the north-eastern corner. This is as we might expect because this is the quadrant for teams that are above-average both offensively and defensively.
Only a handful of Grand Finalists in either era have finished their home and away season with below-average offensive or defensive ratings. And, in the modern era, just two teams have gone into the Finals with below-average defensive ratings - Melbourne 2000 and Port Adelaide 2007, both of which finished as runners up in their respective seasons.
Melbourne finished its home and away season conceding 100 points or more in 4 of its last 7 games, and conceding 98 and 99 points in two others. Those results took a collective toll on its defensive rating.
Port Adelaide ended their 2007 home and away season more positively but probably not as well as a team second on the ladder might have been expected to – an assessment that seems all the more reasonable given the Grand Final result just a few weeks later. In that 2007 Grand Final, Geelong defeated them by 119 points.
The chart for the modern era also highlights a few highly-rated teams that could consider themselves unlucky to have not made the Grand Final in their years – the Adelaide 2016 and St Kilda 2005 teams in particular, though that Saints’ rating was somewhat elevated by its 139-point thrashing of the Lions in the final home and away game of that season.
Based on the relatively small sample of successful teams shown in this chart, it’s difficult to come to any firm conclusions about the relative importance of offensive versus defensive ability for making Grand Finals and winning Flags, and impossible to say anything at all about their relative importance in getting a team to the finals in the first place.
To look at that issue we use the ratings in a slightly different way. Specifically, we use them to calculate the winning rates of teams classified on the basis of their offensive and defensive superiority or inferiority at the time of their clash.
Those calculations are summarised in the table below, which also groups games into eras to iron out season to season fluctuations and make underlying differences more apparent.
The percentages that are most interesting are those in the left-most column in each block.
They tell how successful teams have been that have found themselves stronger defensively but weaker offensively than their opponents.
What we find is that, in every era since WWII:
- in home and away games, teams that were superior defensively and weaker offensively have won slightly more than 50% of their games
- in finals, in contrast, teams that were superior offensively and weaker defensively have won slightly more than 50% of their games
We should note though that none of the percentages are statistically significantly different from 50%, so we can’t definitively claim that, in any particular era, defensive superiority has been preferable to offensive superiority in the home and away season or that the opposite has been true in finals. That’s the clear tendency, but the evidence is statistically weak, so the differences we see might be no more than random noise.
In any case, the effect sizes we see are quite small – around 1 to 2% points – so practically it makes more sense to conclude that offensive and defensive abilities have been historically of roughly equal importance to a team’s success in home and away games and in finals.
The Teams of 2017
So, where do the current crop of teams sit?
The chart below maps each of the 18 current teams’ ratings as at the end of Round 12 and the ratings of all 34 Grand Finalists from the period 2000-2016 as at the end of Round 12 in their respective years.
Adelaide stand alone offensively, with a rating almost as good as the 2000 Essendon team who were 12 and 0 after Round 12 having averaged just over 136 points per game in a year where the all-team average score was 103 points per team per game across the entire home and away season. The Dons scored then at a rate just over 30% higher than an average team.
This year, Adelaide are averaging just under 119 points per game in a season where the all-team average is just under 91 points per game, which is also about 30% higher. They are, clearly, a formidable team offensively, though they’ve yet to impress consistently defensively.
The 2017 Port Adelaide and GWS teams come next, both located just outside the crop of highest-rated Grand Finalists, and having combined ratings a little below Adelaide’s. This week’s loss to Essendon had a (quite reasonably) significant effect on Port Adelaide’s rating, as did GWS’ loss to Carlton.
Geelong, Collingwood, Sydney, Richmond and the Western Bulldogs are a little more south-east of that prime Flag-winner territory, and would require a few above-expectation performances in upcoming weeks to enter that area. The Bulldogs in particular would need to show a little more offensive ability to push into the group, though they had a similar rating at the same point last season, so who’s to say they need to do anything much more.
Collingwood’s relatively high rating might raise a few eyebrows, but they have, it should be noted, generated more scoring shots in their losses to the Western Bulldogs in Round 1 and Essendon in Round 5, and generated only four or fewer less scoring shots in their losses to Richmond in Round 2, St Kilda in Round 4, Carlton in Round 7, GWS in Round 8, and Melbourne in Round 12. They’re currently ranked 7th on combined rating.
Essendon, Melbourne and St Kilda form the next sub-group –rated slightly above average on combined rating but below almost all previous Grand Finalists at the equivalent point in the season.
No other team has a combined rating that is positive or that exceeds that of any Flag winner at this point in the season since 2000. As such, the remaining seven teams would make history were they to win the Flag.
Still, there’s a lot that can happen between now and the end of the season, as we can see in this final chart, which shows 2017 team ratings and the ratings of all non-Grand Finalists from the seasons from 2000 to 2016.
There are plenty of sides in the chart that were rated very highly at the end of Round 12 that never got as far as Grand Final day.
For example, the Geelong 2010 team was 10 and 2 after 12 rounds, one game clear at the head of the competition ladder with a 156 percentage. That team went 7 and 3 over the remainder of the home and away season to finish runners up in the minor premiership before being eliminated by the minor premiers, Collingwood, 120-79 in a Preliminary Final.
And, in any case, in a year where results have constantly surprised and where two wins currently separates 5th from 17th on the ladder, no team can reasonably feel assured of progressing into September, let alone to the MCG on the 30th.