Friday, September 11, 2009

Goal Differential and Points - Part Deux

In a previous article I discussed goal differential and how well it explains how many points a team gets in a season. I would highly recommend reading that article for those who haven't. The short of it is that goal differential almost perfectly explains how many points a team gets. Other than goal differential, how well a team does in close matches compared to blowouts determines how many points they will get. If a team wins a lot of games by a single goal and then gets beat several times by 3 or more goals then they will over perform. In other words, they will get more points than there goal differential indicates that they "should" according to their goal differential.

The question at hand is whether this is based on luck, skill or perhaps something else entirely. Here are some possible explanations.


The main argument for luck is that it plays a role in every single goal that is scored. The difference between a perfectly played ball and one that goes wildly into touch is probably less than a millimeter in where the foot strikes the ball or a tiny change in the amount of force applied. The team aspect of the game also throws an element of luck into the build up and scoring chances. Sometimes your perfect through ball will be run onto by your teammate, other times he won't get there because he was expecting you to play it directly to him. The referee also adds some variance. Sometimes the linesman will miss a player just offside, other times he'll get it right and on occasion he'll rule that a player that was onside was off.

Over the course of a season, these things will even out so that the number of goals scored and allowed will be pretty close to representing how much skill teams have at each end of the field. On the other hand, luck still will have played a big role in the timing of those goals. Some matches it will all be clicking and you'll beat a team by 3 goals, netting two you didn't really need. Other days you'll play an inferior opponent but will be held to a 0-0 draw because the bounces weren't going your way. If things happen to go your way when the score is close more often than they do when you are well ahead or behind then you will do better than teams with the same goal differential and vice-versa.

Luck certainly plays some role, I don't think anyone could reasonably argue otherwise. Whether it's a small factor or the only thing that matters is impossible to prove as it is the null hypothesis. In other words, we can only show that luck is the only factor by showing that nothing else could explain it.

Player Skill

The argument for player skill is that in addition to football skills there is a mental skill that allows some players to play better than they usually do (or not as much worse as usual than others) in more important game situations. In American sports, this is often referred to as a player being clutch. In soccer this could appear a number of ways. The most obvious example is penalty kicks. When you're watching it certainly seems like some players are better than others at not letting nerves get the best of them and just burying it as they should. In the run of play, you could see this if an attacking player is more likely than others of the same skill level to score in the dying minutes when his team needs a goal to even the score or go ahead.

Unfortunately, because football is a team sport evidence of this is going to be hard to come by. Perhaps one could look at attacking players' scoring rates in various situations. That doesn't seem like it would work though as there would be serious sample-size problems and it would be hard to untangle other possible explanations like tactics and the play of teammates.

Manager Skill

Differences in tactics could also play a role. Suppose a manager has a tactic that drastically improves his team defensively, but that also makes them unlikely to score. If he is ahead by a goal near the end then he will use it and as a result his team will give up fewer equalizing goals when they are ahead. They will also score fewer goals in that spot. This has a double effect. They will perform better than the average team of their skill level in close games and also score fewer useless goals making their goal differential less than other teams of their skill level. This will cause them to over perform relative to their goal differential. Similarly, if a manager has a tactic that drastically increases his teams chances of scoring but also being scored on then the same will hold because they'll draw even more often when behind in a one-goal match and will also give up more extra goals.

This and player skill is difficult to untangle, but the argument could be made that a manager skill is picking players who are able to step up in important situations. To look for this, I will compare how well different teams perform compared to their goal differential through various seasons. If the manager or group of players is important then some teams should consistently outperform those with the same goal differential.

Other Factors

The above explanations focused on doing better or worse than teams of a similar skill level in close games. It could also be the case that some teams do better or worse than those of the same skill level when they are well ahead or behind another team in a match. On the positive side of things, if a team has a lot of competition for starting spots then the players might be more willing to play with full effort even when the result has been decided in order to impress the manager. If this is the case, then they will get more goals on average when they are already ahead by two or three goals and will underachieve compared to their goal difference. Another way to think about it is that their goal differential would be higher than their actual skill level so they would get fewer points than those with a similar difference in goals. On the negative side, a team may give up more easily than others. This could be due to a lack of mental toughness or players smartly backing off a bit in order to conserve energy for the long season and prevent injury. To clarify, it not only has to be the case that a team does this, but they would have to do so more than the average team for it to matter.

Future Work and Discussion

The next article in the series I will look into these explanations and examine whether some teams seem to perform better in important situations than others with similar skills.

In the meantime, I'd love to know your opinion. Do you think it's all luck or is player and manager skill important? Do you think it might be something else that I didn't mention? Please give your thoughts below.


  1. I have no empirical evidence to back up my opinion but I have to think that luck is overwhelmingly the most important factor here. The whole player skill, or clutch argument is I think largely a myth propagated by average fans and the media based on insignificant sample sizes and the human tendency to want to explain things as something other than randomness/luck/variance. The flipside is I guess the 'big game player' criticism where (as Bjorn so often likes to point out) some otherwise elite player is criticized for not performing in big games when again you are talking about tiny sample sizes (often all it takes is to not perform in one big game and all of a sudden the player isn't a big game player).

  2. Thanks for the comment.

    I think a good example of what you're talking about with reputations is Zlatan. In one my UEFA articles I mentioned and linked to a youtube clip of a goal of his for Sweden. He scored in the dying seconds giving Sweden the win and two more crucial points. It was a breakaway play and he had only one chance because even if it had been a corner the ref likely would have whistled the game over. Scoring there probably doubled Sweden's chances of making it to the World Cup finals. It's hard to imagine a more important situation and he put it away.

    It's just one goal, I'm not saying it means he's a clutch player or anything like that but he has a reputation for not showing up in big games and important situations. I agree with your point. If he'd scored that goal four years ago I doubt he would be viewed that way.

    Another factor with big games is that they tend to feature teams that are very strong defensively. It's harder for everyone to score and and teams are getting better and better at limiting attacking chances.