Introduction
This is a first in a series of posts that I’ll do
about statistics in the Australian Football League. The posts will be about
which AFL statistics I think matter – that is, what I think they tell us about
how AFL teams and players go about scoring and stopping the other team from
scoring.
Yes, there is a lot of writing out there already about
AFL that uses statistics and numbers, and a lot of good writing. This is just how all of those statistics make sense
to me. I hope if you’re reading this you find something in here that’s useful
for you too.
A lot of the thoughts I’m going to talk about here
came about as a result of me trying to devise a method of rating AFL players,
without having access to the detailed data that Champion Data use to devise
their ratings. We may still get
to that in the end. It turns out though that to work out how each player
contributes to winning a game you need to first think about how teams go about winning them.
Points differential:
the most important statistic of all (duh…)
In their AFL Prospectus 2018, Champion Data made this
obvious but important point:
“… we are asked [:] What’s the most important stat? As
respectfully as possible we answer with POINTS FOR. It’s the one stat that
guarantees a win … We go on to explain it’s more about how you get to that
point.”
Of course points for compared with points against is
important. There is a position here though that may not be quite so obvious. Some
would argue that only the win or the loss matter, and not the margin of victory
or defeat. Margins
though tend to be a better predictor of future performance. Close wins may
bring exhilaration and relief, but in general a team should take more comfort
out of a comfortable win than a close one.
Metres gained matters
Metres gained gets some bad press, perhaps because it
sounds like ‘a stat too far’. An article on The Roar last year even went so far as to call the statistic
‘irrelevant’. The main argument was that it doesn’t take into account the
outcome of the possession – a long kick to the opposition would be credited
with many more metres than an effective handball backwards to a teammate. “While it is impressive to see a
player average over 300 metres gained a match,” the author says “the statistic
is mostly empty in its meaning.”
The article makes some good points, but I disagree
that metres gained is irrelevant. Indeed to me, there is hardly anything more
fundamental to performing well at Australian rules football than gaining
metres. When you’re watching your team, apart from when they’re actually in the
action of kicking goals, what do you most want them to do? You want them to GET
THE BALL CLOSER TO THEIR GOALPOSTS and GET THE BALL FAR AWAY FROM THE OTHER
TEAM’S GOALPOSTS!
Now a critic of metres gained may point out that it
isn’t so great if you kick 50 metres straight to the opposition. That’s true –
‘effective’ metres would probably be a better measure. We’ll get to more about
keeping possession later.
Kicking the ball 50 metres to the opposition though isn’t
necessarily a horrible outcome, despite what I will say later on about the
value of turnovers. Now if the opposition run the ball down the field and score
a goal off your turnover that is obviously a bad result. That worst-case
scenario is relatively uncommon though – only about 10 per cent of
possession ‘chains’ end in goals, and on average those ‘chains’ only last for
about three disposals and gain about 45-50 metres. In other words, even if you
kick 50 metres to the opposition it’s unlikely the other team will punish you
by running the ball down the field and kicking a goal (obviously depending on
where the ball is, and how badly you butcher the kick). More likely is that the
ball may come back to your team within a few possessions, and back around where
you started.
What about an effective kick across the ground that
gains no metres but sets the team up for a shot at goal? Isn’t it true that
metres gained isn’t very good at accounting for that? That cross-ground kick however
is only valuable if the TEAM gains metres on a subsequent possession. If the
opposition stops the ball before it goes any further then it’s just a kick
across the ground that didn’t help much. The objection to metres gained here is
more about crediting the total metres gained by the team to the individuals in
that team – how much did that cross-ground kick help the team to score? – not about
the value of the total metres gained by the team itself.
Metres gained matter. In each of the four AFL seasons
since the statistic was made public in 2015 no other relatively common used
statistic – except statistics directly related to scoring, e.g. score
involvements and goal assists – has been more positively correlated with points
differential (see table below).
Good teams like West Coast and Richmond had less
disposals last year than their opponents, and lower teams like the Bulldogs and
St. Kilda had more. Richmond were smashed in hit outs and clearances, and West
Coast were behind on tackles. The higher teams though almost always had
positive metres gained differentials over the season (see table below). It is
one of the few relatively common statistics that you can reliably
count on good teams being ahead in.
[EDIT: Metres gained differential in a single match should, by definition, ALMOST reflect goal differential. Nevertheless, metres gained are still highly correlated with scoring.]
Once you view gaining territory as an important
indicator of a football team’s ability to score goals, the importance of some
other AFL statistics falls into place. Inside 50 entries – a statistic that has
been noted by others to be highly correlated with winning – indicates metres gained by measuring the number of
times a team gets the ball past a particular point on the ground, one it has to
pass over in order to score goals. An inside 40 or inside 30 measure would also
indicate this.
(Rebound 50s indicate metres gained as well, but since
they indicate metres gained in a team’s defensive part of the ground they are
negatively correlated with winning. Put inside 50s and rebound 50s together and
you get some of the way to a decent proxy for metres gained.)
It also indicates why the number of kicks that a team
or player records is generally important for winning (other than, of course, it
is the only way to score goals), and why one kick is generally more important
than one handball – more kicks often leads to more territory gained. Conversely
though it also explains why just amassing kicks sometimes does not lead to
success; for example, two short kicks of 20 metres get a team no closer to goal
than one long kick of 40 metres.
All of this is not to say however that an individual player’s contribution to
their team’s performance should be primarily measured by metres gained. The AFL
leaders in metres gained per game last year were Jayden Short, James Sicily,
and Nathan Wilson. Nobody thinks that these are the very best players in the
league. (Maybe they should … but probably not.) That is because what these
players don’t do as much as some other players is win their teams the ball in
the first place.
Turnovers are the main
source of scoring shots
In Australian rules football, the significance of
individual possessions in helping a team score can sometimes seem hard to work
out. The ball can pass back and forth between teams several times before anyone
has a legitimate chance to score. Teams can also have very different styles
when in possession of the ball, with some teams preferring a ‘high-possession,
low-risk’ game, and others preferring to be more direct.
Let’s try and simplify it then. Obviously if you have
possession of the ball you are the only team that can score until your ‘chain’
of possessions is broken. We’ll call any unbroken sequence of possessions by a
team – whether of one possession or ten – a ‘possession chain’.
Let’s say that a possession chain for a team can start
in one of three ways:
- the team gets a ‘clearance’ – i.e. it clears the ball from a ball-up, either from a stoppage or a centre bounce at the start of a quarter or after a goal
- the opposition turns the ball over; or
- the opposition scores a behind, giving the team a kick into play.
Let’s also say that a possession chain ends in one of
three ways:
the team scores a goal or behind – a successful (or at least partially successful) possession chain;
the team turns the ball over; or
the possession chain is ‘stopped’, due to a ball-up, or because the quarter is over, or because the scoreboard is on fire – basically any unsuccessful possession chain that does not result in the ball going directly back to the opposition.
(That may not be completely technically correct
according to how Champion Data defines these terms, but I think it’s close
enough for the purpose of my main point here.)
Therefore, for a team we will say that:
Possession chains started =
Possession chains ended
Clearances + Opposition turnovers +
Opposition behinds = Scoring Shots + Turnovers + Stopped Possession Chains
Or, more importantly:
Scoring Shots = Clearances + Opposition
turnovers + Opposition behinds - Turnovers - Stopped Possession Chains
For example in 2018 premiers West Coast averaged 24.7
scoring shots per game. By the definition above they started 117.9 possession
chains per game, from 36.6 clearances, 71.6 opposition turnovers, and 9.7
opposition behinds. Of the 93.2 unsuccessful possession chains they had per
game, 67.7 of them were turnovers, and 25.4 were stopped possession chains.
The bottom team in 2018 Carlton averaged 17.9 scoring
shots per game. By the definition above they started 109.7 possession chains
per game, from 35.0 clearances, 62.3 opposition turnovers, and 12.4 opposition
behinds. Of the 91.9 unsuccessful possession chains they had per game, 67.3 of
them were turnovers, and 24.6 were stopped possession chains.
What’s the main difference in the possession chains of
those two teams? Clearances, behinds, and stopped possession chains are all similar.
The main difference is West Coast started more chains through opposition
turnovers – about the same difference as the difference in scoring shots.
Returning to our scoring shot formula above, let’s now
look at scoring shot differentials, or scoring shots for the team less
opposition scoring shots. With a bit of mathematics we can show that:
Scoring shot differential =
Clearance differential – 2 * Turnover differential – Behinds differential –
Stopped possession chains differential
Maybe it’s just me, but I found this really
interesting when I worked it out. Turnovers are not only more common than those
other components they count for twice as much in this equation. When there is a
stoppage one team’s possession chain ends, but then each team has about a
50 per cent chance of starting the next possession chain. When the ball is
turned over, one team’s possession chain ends and the other team’s begins.
Turnover differential does not explain everything, as
there are other ways to start possession chains. 2018 runners-up Collingwood gave
up a lot of turnovers, but they were good at getting clearances. 2016 premiers
the Western Bulldogs were fantastic at winning clearances. It is just that it
is less likely to get a high differential through clearances or stoppages rather
than turnovers as there are less of them. Hence, clearance differential is less
correlated with winning than turnover differential is.
In terms of valuing players this suggests that not
only are players with high clearance numbers such as Tom Mitchell and Nat Fyfe
important for starting possession chains, but so are defenders who get a high
number of intercepts such as Alex Rance and Jeremy McGovern. Though the
question then is how important is the individual player who records the
clearance or intercept to the team getting possession? How much of the credit
should go to the ruckman getting the hit out, or the structure of the team
defence?
Given how fundamental I said metres gained were how then
does turnover differential relate back to that measure? If the main aim of
Australian rules is to score by getting the ball close enough to your goal to
do so, then the way to progress the ball down the ground is to get possession
of the ball and keepisng it. The best possessions are those that help the team
to gain a high number of metres with a relatively low risk of giving the ball
back to the opposition, or of the ball being ‘stopped’. On defence, your aim is
to stop the other team doing this.
Sounds simple, right? Well, it’s easier said than
done. Also, what may be complex about Australian rules is the many ways you can
go about doing this.
Goal accuracy: converting
those shots at goal
Another statistic that I think is important is goal
accuracy, though not quite in the same way as the other main statistics I have
covered above.
To state the obvious, six behinds are needed to gain
as many points as a single goal. Given that the difference between premiers
West Coast and last-placed Carlton was only about seven scoring shots per game
you want to convert those chances when you get them. Or given that goal
accuracy tends to even out over the season, you want to convert those chances
when they are most important. (West Coast almost didn’t in last year’s Grand
Final, until Dom Sheed
did.)
Conclusion
In Australian rules football the ways for teams to
score are:
get possession of the ball, and keep it;
gain metres while you have the ball in order to get a shot at scoring; and
get as many of those shots at scoring through the goal posts as possible.
For defence the ways to stop the other team from
scoring are:
try to get back possession of the ball, or otherwise get a stoppage;
if you haven’t got the ball back yet stop the other team from getting the ball down the field far enough to have a shot at scoring; and
if the other team does get a shot a scoring, try and limit the chance that it is a goal.
This is essentially how I
am going to talk about the value of AFL statistics in this series. I’ll mainly
evaluate teams and players by how good they are at doing these things. I also
plan to talk about how the evolving nature of statistics has given us a better
picture over time of the effectiveness of teams and players. Some conclusions
will be obvious, but some may be less so. In the end, we may yet even get to that 'player rating' system.
