Monday, September 14, 2009

Is Performance in Close Matches Luck or Skill? (Goal Diff and Points - Part 3)

Note: Here are parts one and two of this series. I strongly recommend reading those before reading this, though I give a summary at the beginning if you just want to get to the business of which teams have gotten lucky and/or been clutch over the last several years.

In the first article I discussed how well a team's number of league points at the end of the season corresponds to their goal differential. The answer was extremely well. In the second article I discussed why a team would over or under perform relative to their goal differential. This comes down to how good their results were in close matches compared to teams that are similarly skilled, at least as judged by their goal differential. I then discussed why some teams would getter better results than others.

The question at hand is whether there is a "close-match skill" that causes some teams to play better when it matters most compared to teams of the same skill level otherwise.


To test this, I used the same data set as before. It includes every season of the Spanish Primera Division from 1987-1988 to 1994-1995 and from 1997-1998 to 2008-2009. I also have the English Premier League for all seasons since 1995-1996 and the Serie A since 2004-2005. I believe this to be all seasons for the three leagues in which they had 20 teams. For the first 8 seasons of La Liga I converted the points so that a win would be worth 3 points as it is today instead of 2 as it was then.

Using this data I looked at the correlation between how many points above or below goal-differential expectation each team gets a given season and the next season. In footballing terms this statistic will tell us if a team's performance in close matches is consistent from one season to the next. A large positive figure would indicate that teams doing particularly well in important situations this year will tend to also do so next year. A large negative value would mean that teams that do well for their skill level this year will tend to be bad at that next year. A value close to zero would tell us that performance in close matches this year means nothing as far as next year goes. In other words, values close to zero mean that there is no evidence that performing especially well in crucial situations is anything other than luck.

Here is the correlation coefficient for each of the three leagues:
English Premier League: -0.0218 (sample size 240)
Spanish Primera Division: 0.05579 (sample size 357)
Italian Serie A: 0.02312 (sample size 73)

As you can see these values are all very close to zero. In fact, the correlation coefficient for the Premiership is negative! For the fellow nerds out there, the p-values for the one-sided t-test were 0.147 for the Spanish league and 0.423 for the Serie A. The correlation for the Spanish league is in the ballpark of being statistically significant but is not at the usual 5% level, or even at 10%. Given the large sample size, it's safe to conclude that the correlation is effectively 0. There is no evidence that teams overperforming their goal-differential expectation can be attributed to anything other than luck, or at least anything that would carry over from one season to another.

A Look At Individual Clubs

While that is very conclusive, let's look at how different teams have done compared to their goal-differential expectation. For all teams in England with at least 10 seasons in the sample, here is a chart with the average difference between actual points and goal-differential expectation each season.

Liverpool have gotten an average of 1.5 fewer points than their goal-differential suggests that they should. At the other end, West Ham have managed to outperform their expectation by nearly 3 points a season. That Manchester United have done better than expectation might be the least shocking thing I have uncovered in all of my research for this blog.

Looking at the numbers, I think the chart is more evidence that there isn't skill involved in outperforming your goal-differential - that it's just luck. One reason for this is that the top teams are all over the place. Admittedly, Manchester United are near the top and are easily the best club in England over the sample. Other than them though, Chelsea and Arsenal have run close to average, while Liverpool are well below average.

On the flip side, West Ham United are the team that has been most successful compared to their skill level! While they haven't been the worst club to see the top flight, a look at them over the years of the sample gives one no reason to think that they have something that makes them the best team in close matches compared to their quality. They went through 4 managers during that time and were relegated. The relegation isn't really relevant as absolute level isn't what we're looking at but how well a team does in close games compared to their skill level. To me going through four managers is important. It would be easy to point to Alex Ferguson as the reason for Manchester United outperforming their goal-differential expectation, but then why would a club that sacked four managers in that time do even better compared to their expectation?

Here is the list for La Liga. I included all teams with at least 10 seasons in the sample:

Once again, we have a similar pattern. Strong teams such as Athletic Bilbao and Real Madrid are near the top but then again there are big clubs like Barcelona and Atletico de Madrid at the other end. Sporting Gijon is similar to West Ham. Again, just glancing at the list of teams that over and underperformed, it seems to be mostly luck, if not all.

For completeness here's the chart for the Serie A:

Just like the others it features a huge club at or near the top and bottom. Reggina plays the role of club that has seriously over performed in close matches for no obvious reason other than luck. Looking at both this and the Premieship data, I found it interesting that Liverpool and AC Milan have under performed in their leagues. Both have a reputation for grinding out wins in cup competitions, most notably the Champions League, even when playing against superior opponents. Apparently those reputations didn't help them any in those spots in league fixtures.


Looking at points earned compared to the goal-differential expectation from one season to the next there is no correlation. Teams that over perform in close matches this season are no more likely to do so next year than those that under performed. In other words, there is no evidence that some teams are able to step up in important situations better than others. Looking at which teams have historically done better or worse than their goal-differential expectation helps to confirm this. While for each of the three leagues there is a team or two near the top that one might think could have this ability to step up, there are other teams at the bottom that it seems should also have it. As importantly, there are teams that have overperformed that have lacked consistency in management and squad so it seems they couldn't have done so if the ability to rise to the occasion were real, or at least an important factor.

Scoring and being scored on are based on a combination of skill and luck, but having the goals come at the right time seems to be all luck.

1 comment:

  1. Very good post, confirming my earlier suspicion.