Exploring the Chance Quality Index: Why more chances doesn’t necessarily mean more goals
Karthik (KV) seeks to establish why more chances don’t necessarily mean more goals.
How do you win a football game? The simplest answer would be to score more goals than the other team. So, how do you score more goals than the other team? Create more chances than the other team and you are likely to score more than them. How accurate is that statement? Not very accurate, in fact. What we can conclude with certainty is that, the team that creates chances of higher quality is likely to score more compared to the other team.
At every press conference that Arsene Wenger has had to attend in the past few years, he would respond to the customary question on possible transfers by stating that he always opts for ‘quality over quantity.’ Signing a player of top quality is more important than signing 3-4 players just to fill the void. We can apply the same principle to chances created. The probability of scoring from a chance of very high quality is more compared to scoring from three chances of mediocre quality. For better clarity, OPTA describes a chance as ‘assists plus key passes.’ Is it possible to measure the quality of a chance? Yes, that is what I have tried to do in the following lines.
The factors affecting the quality of a chance are:
- Distance from the goal
- The angle by which the goal is visible.
- The number of opponent players surrounding the player with the ball.
Based on this, we can say that Chance Quality is:
- Inversely proportional to the distance from goal.
- Inversely proportional to the number of opponent players surrounding the player taking the shot.
- Inversely proportional to angle A as shown.
C.Q.I = cos(A)/D*P
Where, A is the angle between the line joining the centres of the two goals and the line joining the centre of the goal to the point from where the shot is taken. D is the distance between the centre of the goal and the point from where the shot is taken. P is the number of opposition players close to the player when the shot was taken. It is to be noted that Cos of the angle A is taken because, due to the property of the Cos function, as A increases, Cos(A) decreases and that is exactly what we need.
The Ade”Can’t hit a barn door”bayor problem:
If you rewind a few seasons back to the 2008-09 season, you would realize that Arsenal lost 6 games, drew 12 and finished fourth. Teams that we lost to included Fulham, Hull City, Aston Villa and Manchester City. All of them won by a one goal margin (except for Man City) and all the games were dominated by Arsenal, in regards to possession. The number of shots taken by Arsenal also outnumbered the other teams, but the other teams just sat deep and chose the right moment to counter attack and create chances which had a high C.Q.I rating (statistics have shown that 43% of the chances created from transitions get converted to goals). At that time, everyone were busy criticizing the finishing of the Arsenal strikers like Emmanuel Adebayor and the inability of our defensive midfielders to stop perform better and not concede(Alex Song and Denilson), when the actual problem was that Arsenal lacked the creative firepower to breakdown teams and create high quality chances. A study on the recently concluded Barca-Milan game showed that, while Barca dominated possession(65%) and had 18 shots on target compared to Milan’s six, Milan had the best opportunity to score in the game, with a shot that had a C.Q.I rating of 0.08(which looks like a small number, but is actually higher than other shots).
How can the C.Q.I be of help?
A higher C.Q.I pass means a better chance of scoring. So, players who create more number of C.Q.I passes are extremely valuable to the team. For example, in the Barca-Milan game, Xavi, Messi and Dani Alves created chances with high C.Q.I rating and they were also the best players on the pitch. It can also be used to analyse games and bring about changes. Iniesta wasn’t creating much, so Rodrigo Tello came on and immediately created a chance with a fairly high C.Q.I rating. It can even help develop tactics. For instance, we know that counter attacks tend to result in goals 43% of the time, so it makes sense to leave a player that creates high C.Q.I chances high up the pitch (like Messi). ‘The Invincibles’ Arsenal team had great players like Dennis Bergkamp, Thierry Henry and Robert Pires who created chances with high C.Q.I and won games playing counter attacking football.
So often coaches have bemoaned the loss of a game which they felt they had deserved but succumbed because the other team just purely finished their chances. It happened last night with Barcelona claiming they should have won when in fact, they failed to create a better chance than Milan which was fired over by Robinho early on. Indeed, one article after the game questioned why Robinho regularly gets picked even though he hits the target with 44% of his shots and misses some great chances to boot (Ibrahimovic has 60%; El Shaarawy’s, Pato 52%). (As it turned out, the author concluded that goals are not everything as Robinho causes trouble with his movement – and that’s what creates chances for Milan).
While in it’s infancy (give us the funding!), Chance Quality Index has it’s merits if anything to challenge the established conceptions of chances and the likelihood of winning a game. Indeed, it was Wenger who once remarked, “the measure of football is the ratio of chances created to chances conceded” and that he concluded means Arsenal deserve to win the game as they have dominated. This is surely dependent on the quality of chances you create, is it not?
If that is true, however, then Arsenal should follow the route of Barcelona who believe possession is “nine-tenths” of the game. That should ensure Arsenal keep down the number of shots they concede which is currently at 10 per game in the league (and consequently, help them press better) – Barcelona’s is 7 despite both teams creating on average 17 shots per game. But that is patently not Arséne Wenger’s style as he says he’d rather a player who takes in a risk in their passing in the final third than play it safe – for Barcelona, it’s all about the quality of the chance. Wenger prefers urgency and while we are seeing a better drilled Arsenal this part of the season, the fact that they have gone down in half of the games recently, shows there’s gaps in the system.
It’s unfortunate Milan-Barcelona yielded no goals; it seemed the perfect encounter to experiment CQI because it carried with it, the old adage that possession (for Barcelona) should equal chances and consequently, a win – but that’s not necessarily the case. In fact, good chances are harder to come by when defences defend deep as Milan did because the attacking team is hindered by a lack of time and space. Milan on the other hand, had less chances but the best one on the night as Robinho fired over. Had the chance fell to Ibrahimovic, it might have gone in. That’s one of the issues with C.Q.I; it’s still subjective as much as it tries to quantify the art of the chance. Because some players are much more composed in front of goal. Take for instance, Thierry Henry’s goal against Leeds in the FA Cup; for some players, the chance may be harder due to the angle, the defender haring down on Henry’s back and not to mention the technique. But such was the familiarity of him in that position, it was almost a 10 out of 10 chance. It’s notable that in the game against Milan, coincidentally enough, Wenger altered the system so Henry could get into such situations. But it’s probably not the varying expertise of the player taking the shot that’s most important with the Henry chance; sometimes the angled shot IS the optimal way to score.
Back to the goalless draw at the San Siro, Pep Guardiola might have argued that his side deserved to win but Milan might have been the most aggrieved as a shot in the third minute flew over. It proved pivotal.
Filed under: Statistics
Tagged: adebayor, Barcelona, CQI, Statistics, Strikers
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