‘Win the contested ball and you’ll win the game’ – it’s fast becoming a Kingism (calling it contested ball certainly doesn’t help in this regard), but is there anything to it?
Roar expert Cam Rose wrote a great piece questioning the value of this statistic as a predictor of victory a few weeks back; coincidentally a couple of days before I started working on this piece.
What is a contested possession? I put this question to Champion Data around two weeks ago, and didn’t get a response. Presumably my email was considered junk, because it’s not like Champion Data to treat the punters as mugs, is it?
Editor’s note – Champion Data have since replied to Ryan, and it’s in the comments section below. Ryan has also apologised for appearing a tad harsh in the above.
Anyway, based on various definitions I’ve come across, a contested possession (CP) is awarded to a player who ‘wins’ the ball when there are two or more players from opposing sides who have the opportunity to win the ball.
A possession and a disposal are two different statistical classes. They are used interchangeably quite often – I must admit until I’d looked into it I had just figured they were both measuring the same action but in different ways. A possession is a ‘get’, while a disposal is what a player ‘does’ with the ball once they’ve got it.
Ever noticed on the AFL’s stat widgets that ‘possessions’ and ‘disposals’ normally differ by a a handful of numbers?
What I hope to do in this piece is explore the importance of the contested possession as a predictive stat for victory.
Does contested ball matter?
To answer that, lets start with a chart that shows the winning percentage of teams based on ranges of contested possession differentials (a differential being the Cps ‘for’ or ‘won’ take Cps ‘against’ or ‘lost’).
This shows a fairly clear correlation between contested possession differentials and winning percentage. A side is more likely to have won games where it had a positive CP differential. By contrast, a side is much less likely to win in instances where it has a negative CP differential.
Based on the figures, it looks like the positive effect broadly tops out at a CP of +31 – the winning percentage hovers around the 90 per cent mark in brackets above this level (and the same can be seen on the left tail of the curve).
This is fairly intuitive – if a CP is winning a situation where the other team has a chance to win it, and you’re able to win more of these than the other team, then it follows that you’re putting yourself in the best position to win.
“But Ryan,” I hear you utter with some smugness, “correlation does not imply causation”. Yeah, agreed. That’s not what this article is about. Lets now take the analysis a bit further.
If a positive CP stat is important, won’t it mean that a side is more likely to win games even when they end up possessing the ball less often than the other team? Well, actually, this is true: in instances where a team has a negative total possession (TP) differential, they’re only expected to win 25.3 per cent of the time – but win CPs and this expected win percentage lifts to 42.4 per cent. This is a boost of around one third.
The same is true in reverse: if a team wins TP, it can expect to win around 74 per cent of its games. But, in instances where it loses CP but wins TP, the winning percentage drops to 57 per cent – a fall of around 25 per cent.
It’s worth pointing out that the scenario of a team winning contested possessions but losing the overall possession count is reasonably rare. In the 1750-ish games I’ve looked at here, it’s only happened 166 times – less than 10 per cent of games in my sample. In the economics game we call this ‘autocorrelation’, which broadly means something is true mostly because some precondition makes it so.
Here’s a handy graphic that summarises the figures (green means the stage of the equation is positive for winning percentage, red means negative, blue means neutral and grey means not applicable).
This suggests causality, and fairly strong causality. But you’ll notice that there are greens on the ‘up’ side of things, too. This is uncontested possessions, and its something I’ll close on in a couple of paragraphs.
Another way we can test for a causal relationship is to run a test known as a logistic regression or ‘logit’ regression. These are used quite a lot in the medical game as a way of determining survival rates for various conditions.
It works on the principle that you can have two outcomes that can be partially explained by one or more variables: in medicine, its life or death; in footy, it can be wins and losses.
If you hate math, look away now…
You don’t need the details, but the logistic regression of CPs found that for each positive CP differential, a side’s chance of victory increases by roughly 1.5 per cent; although this is on the basis that the ‘starting point’ is a victory change of around 48 per cent.
To put it another, slightly technical way, a team which has a CP of +12 should win around 68 per cent of the time, according to this formula:
1/(1+e0.0704-(0.069653 x 12))
So that’s a bit mathy. But if you want to work out winning percentages based on CP differentials, simply change that 12 to another figure.
To sum up, winning CPs is certainly an important component of success. A couple of different methods of analysis have shown that using differential analysis showed that a positive CP has an influence on winning percentage, regardless of whether a team wins or loses other high level possession categories; while a basic regression test determined a significant causal relationship.
Are uncontested possessions becoming more important?
I’d like to end with a bit of a thought provoker: having a positive UP differential is, increasingly, a marker of success. Check out the below chart, which shows the 200 game moving average winning percentage of teams with a positive CP and positive UP.
In economics, this is what we call “structural change”. Since 2008, the expected winning percentage of teams with a positive UP differential has moved higher by around about five percentage points, while the winning percentage for a positive CP differential have moved lower by around eight percentage points.
Are sides finally cottoning on to the notion that ball use is more important than toughness, and the ability to win the ball in contested situations? Perhaps.
I’d call this ‘The Hawthorn Effect’: after Hawthorn won the 2008 flag, largely on the back of elite skills and smart offensive and defensive structures, the ability to possess the ball more often in situations that allow for the execution of skills appears to be having a strong impact on winning percentage.
That’s not to say contested possessions aren’t important – but we may be moving further into a new era in the AFL where skills and execution are placed at a premium over the ability to bullock through packs and find a way out with the ball.
Karl from Champion Data’s reply to Ryan:
Hi Ryan,
Sorry for not getting back to you but the email went to the wrong people in here. If I saw it at the time we would have replied. Contested possessions are made up of:
* Hard Ball Get (win disputed ball while physically beating an opponent at ground level)
* Loose Ball Get (win other disputed ball at ground level – ie. right place, right time)
* Contested Knock-on (knocking the ball out of a pack to a teammate’s advantage – Hawthorn do this a lot)
* Gather from Hitout to Advantage (similar to looseball get, but the ball was fed from the ruckman’s tap)
* Free Kicks (excluding those paid off the ball or downfield)
* Contested Marks (all marks where an opponent had a chance to mark or spoil – not including marks on lead)In response to Cameron’s comment above (and indirectly to his article) margin does increase with a bigger win in contested possessions but not perfectly. Contested possession differential and final margin have a correlation of about 0.63 this year, or an R-squared of just under 40%, meaning that 40% of the variability in final margins across single matches can be explained by the contested possession differential alone – not bad for a single stat. Obviously other things play a part in the result of games – ball use, pressure, accuracy in front of goal, etc. but contested possessions are a good starting point.
As for the value of a contested possession, theoretically to start with, roughly 10% of a team’s possession chains lead to a goal. Each contested possession is an opportunity to either maintain a possession chain or start a new one by turning over the ball, so with one contested possession you’ve given yourself an opportunity to score (10% of 6 points = 0.6 points net) and have prevented the opposition an opportunity to score (10% of 6 points = 0.6 points net) so one contested possession is worth 1.2 points. Obviously these scoring rates change based on where the contested possession happens, but hopefully you follow the logic.
As for practically – a simple, crude linear regression of margin against contested possession differential puts a value of 1.6 points per contested possession for this season. So no, one contested possession across the course of the game is rarely going to give you a significant advantage, but win/lose the stat consistently or by big margins and you’re in trouble.
A big reason why contested possessions are used so heavily in the media (and I do agree that it is overused as a stat) are because they are also used heavily internally at clubs. Regardless of the volume of the win in the stat it is a simple message that can be relayed to players and one that be easily benchmarked. Players know what it means to beat their opponent and get first hands on the ball more often. There’s no point telling a playing group to go out and kick more goals than the opposition, or to have more shots at goal, or to win more inside 50s, etc.
Gregor
Guest
I haven't seen yet anything elaborating the next-possession-count.That is,if a contested possession, worth,as above. 1.2-1.6 points, leads to another contested possession which goes the other way,that CP is negated.If,on the contrary, that CP leads to a UP, for your side or for the opposition,then you are initiating a scoring chain and increasing the chances of a score.It might therefore be negative. Maybe the CPs should be weighted in some way, to reflect the more and more doomed handballs that Brisbane use, or the freer and freer scope that a good midfield generate with each possession. Contested possessions also allow defensive and forward structures time which may be the point after all.Breaking tags, populating the goalsquare and leading to space all benefit from that time - STBO I know, but it can't just be which team wins the CP count. It has to be what happens next. Your team could win six CPs in a row,like St KIlda, lose possession and have a goal scored against you. A breakdown of areas of conflict, immediate results of actions and upfield consequences would all be useful
Dalgety Carrington
Guest
I think for coaches it is all about the "controllables" and the CP stat would be the ultimate controllable. All factors vary on the scale of controllability. As an example, disposal efficiency is highly controllable in many ways, but is more liable to variables such as talent levels of the play in possession, the availability of safer targets, fatigue, nerves etc etc. While individual brilliance is even more conditional. CP stats are an initial entry point to gain possession of the disputed ball and have greater control over what happens next. The simplest way to look at the object of footy is to score more than your opponents, but another obvious point is to score you need to have possession of the ball first.
Cameron Rose
Expert
I think your last paragraph speaks to an overall point of mine Karl, and that is that the CP count is a lazy throwback or a crutch for coaches. ie - If we just get our contested possession count up, we can win the game. I don't want anyone to mistake my view - of course winning your own ball is important, and we don't want it any other way, but if 18 teams are focussing on winning the CP stat, as they all seem to be, then it actually negates the importance of it, rather than increasing it. Ball movement, run, and the ability to stop the opposition from playing their preferred way (defensive set-ups) are just as integral, but because there aren't any easy stats for them, all we hear about is contested possession.
Karl (Champion Data)
Guest
Loose: 38% (1st Quartile 35% - 3rd Quartile 40%) Hard: 34% (32% - 37%) Free: 11% (9% - 12%) Hitout: 8% (6% - 9%) Mark: 7% (5% - 8%) Knock-on: 3% (2% - 4%)
Karl (Champion Data)
Guest
Video tracking has been investigated. Aside from cost, other issues are: * Inconsistent ground size * Large ground size relative to other sports * 36 players to track (compared to 10 for basketball) * Multiple non-players on the ground * High congestion * High number of collisions It would be a dream to have that level of data detail but it is unlikely any time soon.
Ryan Buckland
Expert
Have you guys thought about incorporating a SportsVU-type of system? Admittedly it would probably cost a bucketload, and may not be fully effective in an area as large as a football stadium, but that could give you access to a range of data that would be very time intensive to capture manually.
Ryan Buckland
Expert
Thank you very much for getting back to me Karl, and taking the time to post a comment. Apologies if I came across as a bit harsh above, but from past experience I don't find Champion Data the most "punter friendly" organisation to deal with. If you're willing to share, what is the rough breakdown of each of those categories as a share of contested possessions? And is it something thats fairly stable, or changes from game to game? I'd imagine the vast majority of CPs are accounted for by Hard Ball Gets? Obviously there are a lot of assumptions built into your analysis (as above); and that's appropriate given we're dealing with an incredibly complex network of cause and effect. But I think your logic is pretty sound. I would be really interested in delving a bit deeper with this with you.
Karl (Champion Data)
Guest
This would be great and it's something we've been asked about in the past. Unfortunately we already have 10 people working on every AFL game and we can't record that level of detail with existing resources. What it would likely take would be a new two-person capture, which over the course of a season is 2 x 3hrs x 207 matches = 1,242 hours of staff costs that need to be paid. That's just not financially viable for what we would get out of the stat. We do record one-on-one marking contests but that is about as detailed as it will get under our existing model.
Cat
Roar Guru
Makes one wonder if a player can get multiple CP's from a single 'rolling maul' type situation. Player grabs the ball only for it to pop free, crawls on knees to rip it away from another player only to have the handball go no where and grab the ball in the scuffle and try to handball through some players legs again. A loose ball get hardly seems contested does it? So that could explain why players seen by most to be outside players can still rack up CP's without getting in and under.
Karl (Champion Data)
Guest
Hi Ryan, Sorry for not getting back to you but the email went to the wrong people in here. If I saw it at the time we would have replied. Contested possessions are made up of: * Hard Ball Get (win disputed ball while physically beating an opponent at ground level) * Loose Ball Get (win other disputed ball at ground level - ie. right place, right time) * Contested Knock-on (knocking the ball out of a pack to a teammate's advantage - Hawthorn do this a lot) * Gather from Hitout to Advantage (similar to looseball get, but the ball was fed from the ruckman's tap) * Free Kicks (excluding those paid off the ball or downfield) * Contested Marks (all marks where an opponent had a chance to mark or spoil - not including marks on lead) In response to Cameron's comment above (and indirectly to his article) margin does increase with a bigger win in contested possessions but not perfectly. Contested possession differential and final margin have a correlation of about 0.63 this year, or an R-squared of just under 40%, meaning that 40% of the variability in final margins across single matches can be explained by the contested possession differential alone - not bad for a single stat. Obviously other things play a part in the result of games - ball use, pressure, accuracy in front of goal, etc. but contested possessions are a good starting point. As for the value of a contested possession, theoretically to start with, roughly 10% of a team's possession chains lead to a goal. Each contested possession is an opportunity to either maintain a possession chain or start a new one by turning over the ball, so with one contested possession you've given yourself an opportunity to score (10% of 6 points = 0.6 points net) and have prevented the opposition an opportunity to score (10% of 6 points = 0.6 points net) so one contested possession is worth 1.2 points. Obviously these scoring rates change based on where the contested possession happens, but hopefully you follow the logic. As for practically - a simple, crude linear regression of margin against contested possession differential puts a value of 1.6 points per contested possession for this season. So no, one contested possession across the course of the game is rarely going to give you a significant advantage, but win/lose the stat consistently or by big margins and you're in trouble. A big reason why contested possessions are used so heavily in the media (and I do agree that it is overused as a stat) are because they are also used heavily internally at clubs. Regardless of the volume of the win in the stat it is a simple message that can be relayed to players and one that be easily benchmarked. Players know what it means to beat their opponent and get first hands on the ball more often. There's no point telling a playing group to go out and kick more goals than the opposition, or to have more shots at goal, or to win more inside 50s, etc.
Cameron Rose
Expert
I think your last comment is the nail-on-the-head one Ryan - they reflect a truth that is self-evident. I reflected in my piece that it's the intangibles of football that are just as important as anything that can be measured, and I stand by it.
Ryan Buckland
Expert
Thats a very good point Brin. In that respect, something I'd like to see is the number of contested possessions a player is awarded divided by the number of contests that player attends. That way you would get a better picture of how "good" a player is in those contested situations, as oppose to simply how "often" the player gets the ball out. There's a difference, a subtle difference, but a difference. Context matters.
Cameron Rose
Expert
It's a fair point Gene, about secrecy and the definition. I can't believe that it would be when two or more opposition players have a chance to win the ball. I was under the impression that it was almost whenever the ball hit the ground, plus through any stoppages, contested mark, etc. Here's a definition from The Age in 2011 - "A player's contested possession tally is the sum of his hard-ball gets, loose-ball gets, contested marks, free kicks for, gathers from hitouts and contested knock-ons" It must be noted that loose ball gets count. Does this mean if I kick it to a bloke 20m clear in space, but it gets to him on the bounce, it's a CP? My reading is yes. From Ted Hopkins, who founded Champion Data, in 2012, also from The Age - Contested possession is overrated as a measure of a good football team, was misunderstood by many in the game and had become ''one of the fashion statements of the time''.
Ryan Buckland
Expert
Thanks for the comments guys. I didn't want to delve too far into theory, but ok here we go. Its self-evident that a team who "wins" the contested possession stat has a better chance of victory. Think of it this way: if a contested possession is awarded to a player who collects the ball when there is an opportunity for the other team to also get the ball, the player is therefore denying the other team an opportunity to use the ball. Win more of these opportunities than the other team, and you're putting yourself in a good position to play the game the way you want to play it. So in that respect, CPs likely reflect something that is self-evident: having the ball more often than the other team is a good predictor of success. Total possessions are possibly a good indicator of this, but CPs take it that one step further because they reflect not only getting the ball, but denying the opposition a chance to get the ball. But, what probably counts more is what you do with the ball once you've got it. This is where things like structures, skills and speed come into play. As much as I'd like to think you can quantify everything, the truth is in footy its really difficult to get something down to one number. There are so many factors at play - not least of which is "geographic information" (where players get the ball, where free kicks are granted, where marks are taken, where ballups occur) which is a largely untapped area of research as far as I know. But (another but); that doesn't mean stats are meaningless, and hopefully what I've shown above is that contested possessions are a pretty good indicator to track - even if they aren't the be-all-and-end-all they are made out to be, and even though they probably reflect a truth that is self-evident.
Stuart
Guest
I agree Brin. I realise that someone with a statistical background is always going to want to run an analysis to prove or disprove something based on measurable figures, but the reality is exactly as you mention. I would go a step further and say that you can't use the results in any meaningful way. It comes down to a very basic principle, if the ball is not our teams hands, we need to get it or we can't win. If I don't have to fight for it, good, but if I do, I will need to go in with more intent than my opponent. It really is not rocket science.
Cat
Roar Guru
Hmmm interesting, if CP is indeed 2 or more opposition players having an opportunity to win the ball, what is a 1v1 considered? What exactly qualifies as 'an opportunity to win the ball'? Really wish we could score an interview with someone at Champion Data, far too much is kept secret and I often wonder if the behind the scenes measurements of these stats aren't tweaked occasionally without anyone knowing. Is a CP from 2000 measured exactly the same as a CP from 2013 or has something changed in an interpretation of 'a chance to win the ball'? With so many changes to umpiring interpretations I think its fair to wonder how much, if any, interpretation there is in stats.
Cameron Rose
Expert
I had to read over that a few times to get my head around it, and still not sure I'm there yet! Needs to be further dumbed down for the likes of me! I still maintain that it is the team that is playing the better football and winning the game that fuels most of this statistical dominance, rather than the other way around. The reason I don't buy into it is because if it is a true indicator of success, based on the way football is played, then the contested possession differentials should increase with the margin, but they don't. Carlton only won it by 10 against St Kilda on the weekend (they won by 14 goals), and Adelaide only won it by 13 against GWS (68 point win). We saw that the Saints won it by 2 against Geelong the other week despite losing by 100 points. It tells me that the game is actually won and lost outside the contest. Watching St Kilda kick to Nick Riewoldt on the weekend, it's not hard to see why I would think that's the case. The Saints may have won three CP's in a row before getting the ball to a teammate in space, but if he can't hit a leading player 10m in the clear, and in fact kicks it over his head to the opposition, then that +3 count is irrelevant. Considering how important rebound from the defensive half is, I'd suggest contested possessions won in the back 50 are the most important, but then most teams have numbers back, which should help in that regard, so almost every team will have a positive differential in that part of the ground. Interestingly, the Dogs smashed Geelong in CP's (+37, which is about as high as it gets I reckon), and Port were +17 against Essendon, and both lost. However, the Cats and Dons both had significantly higher tackle numbers (+34 and +33), which may mean that applying pressure to a contested possession winner is more important than the win itself. Anyway, great piece Ryan, I applaud anyone who wants to dig deeper and doesn't just take for granted what we hear from the media.
Brin Paulsen
Roar Guru
Interesting article Ryan. Thanks. While you've shown that there is real data to underpin the importance of contested possessions I've always believed that the growing profile of the CP count (aside from King's love affair with the term) is due to it being one of the few stats that coaches can choose to use as an indicator of effort. Players can rack up disposals and uncontested possessions just by standing in open space. From a coaching perspective, CP can give you a real, tangible idea of whether your team is putting in if your blokes are winning the direct, head-to-head contests against opposing players. It's a new-world stat for an old world idea. The coach says "Do your job, show me you want it more, beat your man one-on-one to the ball". CP is the closest statistic we have to showing whether a player is putting in the effort and doing that.