Thursday, June 26, 2014

Football and the (mis)use of statistics

I have found a second reason to get interested in football (*): it can help teach students basic lessons about probabilities and more generally the use and misuse of statistics. Here is an example taken from Nate Silver's website, fivethirtyeight.com, (my emphasis in the text):

(*) As in this previous post, it does nor require watching the games, thankfully ;-)

(...)
Reep’s research was quite groundbreaking for its time, even if it was fatally flawed. The Wing Commander gathered data on how often a given number of successful passes were strung together, and how frequently goals resulted from those sequences, broken down by length. Reep determined that a team’s probability of retaining possession dropped precipitously with each consecutive pass attempt, and that most goals were scored on possessions of fewer than three passes — often originating from quick counterattacks.
In Reep’s mind, this meant teams should abandon trying to control possession and maneuvering through the defense with endless passing. Instead, they should focus on getting the ball downfield in as few movements as possible on offense, and applying pressure on defense to generate opportunistic counter-rushes. The numbers seemed to suggest that the long game was the most efficient tactic for soccer success.
But subsequent analysis has discredited this way of thinking. Reep’s mistake was to fixate on the percentage of goals generated by passing sequences of various lengths. Instead, he should have flipped things around, focusing on the probability that a given sequence would produce a goal. Yes, a large proportion of goals are generated on short possessions, but soccer is also fundamentally a game of short possessions and frequent turnovers. If you account for how often each sequence-length occurs during the flow of play, of course more goals are going to come off of smaller sequences — after all, they’re easily the most common type of sequence. But that doesn’t mean a small sequence has a higher probability of leading to a goal.
To the contrary, a team’s probability of scoring goes up as it strings together more successful passes. The implication of this statistical about-face is that maintaining possession is important in soccer.2 There’s a good relationship3 between a team’s time spent in control of the ball and its ability to generate shots on target, which in turn is hugely predictive of a team’s scoring rate and, consequently, its placement in the league table. While there’s less rhyme or reason to the rate at which teams convert those scoring chances into goals, modern analysis has ascertained that possession plays a big role in creating offensive opportunities, and that effective short passing —fueled largely by having pass targets move to soft spots in the defense before ever receiving the ball — is strongly associated with building and maintaining possession.
(...)

Tuesday, June 17, 2014

Le régime des intermittents du spectacle: injuste, inefficace, coûteux, sensible aux abus et pas vraiment contrôlé...

C'est un bon moment pour (re)lire le rapport 2012 de la Cour des Comptes intitulé "Le régime des intermittents du spectacle : la persistance d’une dérive massive".

Voici quelques morceaux choisis:

En 2010, les cotisations s'élevaient à 232 M€, les prestations servies à 1263 M€, pour un déficit de 1013 M€. En d'autres termes, le déficit est de 4.5 fois les cotisations, ou encore, les cotisations ne couvrent même pas 20% des prestations...

En 2010, ce déficit "a représenté un tiers de celui de l’assurance chômage dans son ensemble, alors même que les intermittents ne représentaient que 3 % des demandeurs d’emploi en  fin d’année. 
Sur les dix dernières années, le déficit cumulé du régime des intermittents s’est établi à un montant proche de l’endettement total du régime d’assurance chômage (9,1 Md€ à la fin 2010). 

Quant à l'aspect redistributif: "80 % des intermittents avaient un revenu annuel supérieur à 18 110 €, soit un niveau proche du salaire annuel médian net dans la population française (19 158 € en 2008 selon l’INSEE)."

Et pour ce qui est des abus: 

"Fondé sur un système entièrement déclaratif, le régime des intermittents du spectacle apparaît particulièrement vulnérable à la fraude." 

"La « permittence » désigne la pratique selon laquelle des intermittents sont employés de manière permanente ou quasi permanente par un même employeur. Dans son rapport de 2007, la Cour avait montré que cette pratique, loin d’être marginale, concernait 15 % au moins des bénéficiaires des annexes 8 et 10."

"Le recours à ce type d’emploi permet de majorer les revenus  des salariés grâce à des allocations nettement plus favorables que celles résultant de l’application du droit commun. Une étude réalisée en mars 2008 par l’assurance chômage a ainsi montré que « l’allocation moyenne perçue par un technicien déclarant plus de 1 600 h par an était de l’ordre de 3 fois  supérieure à l’allocation qu’il aurait perçu s’il relevait du régime général dans le cadre d’une activité réduite. Cette allocation devenait 9 fois supérieure lorsque le technicien déclarait entre 936 h et 1 600 h ". 

Le système de contrôle?

"S’il permet de tester la cohérence des renseignements respectivement fournis par les employeurs et les salariés, ce système de contrôle reste inopérant dans les cas de collusion entre employeurs et salariés. Rien ne permet en effet de s’assurer que les renseignements transmis d’un commun accord par un employeur et son salarié correspondent à la réalité. Or un tel risque ne peut être écarté dans la mesure où les salariés, comme les employeurs, peuvent avoir un intérêt commun à reporter sur le régime des intermittents du spectacle le financement pour les uns d’une partie de leurs revenus et pour les autres d’une fraction de leur masse salariale."


Bref, un système injuste, inefficace, coûteux, sensible aux abus et pas vraiment contrôlé...

Monday, June 16, 2014

Football & Economics

So, apparently, there is at least one reason to watch football games this month:



Photo
CreditOlimpia Zagnoli
Continue reading the main storyShare This Page
Continue reading the main story
THE World Cup is finally underway. At long last, soccer fans can don their team colors, head down to the local pub — and begin collecting data to test economic theories.
Or at least, some of us will.
For instance, I’m interested in penalty kicks. In addition to being an exciting part of the game, penalty kicks present an opportunity to test an important idea in economics: the Nash equilibrium.
The economist John Forbes Nash Jr. analyzed how people should behave in strategic situations in which it is not optimal to repeatedly make the same move — like the children’s game rock, paper, scissors, in which selecting one move again and again (rock, rock, rock ...) makes you easy to beat. According to Mr. Nash’s theory, in a zero-sum game (i.e., where a win for one player entails a corresponding loss for the other) the best approach is to vary your moves unpredictably and in such proportions that your probability of winning is the same for each move. In rock, paper, scissors, for example, the optimal strategy is to mix your choices randomly among the three options.
To test this theory in the real world, we can study penalty kicks, which are zero-sum games in which it is not optimal to repeatedly choose the same move. (The goalie has an easier time stopping your shot if you always kick to the same side of the net.) Unlike complex real-world strategic situations involving firms, banks or countries, penalty kicks are relatively simple, and data about them are readily available.
I analyzed 9,017 penalty kicks taken in professional soccer games in a variety of countries from September 1995 to June 2012. I found, as Mr. Nash’s theory would predict, that players typically distributed their shots unpredictably and in just the right proportions. Specifically, roughly 60 percent of kicks were made to the right of the net, and 40 percent to the left. The proportions were not 50-50 because players have unequal strengths in their legs and tend to shoot better to one side. Shooting 50-50, in other words, would not take full advantage of their better leg, while shooting any more often to the stronger side would have been too predictable.
In accordance with Mr. Nash’s theory, penalty kicks shot to the left were successful with the same frequency as kicks shot to the right — roughly 80 percent of the time.
 Penalty kicks are just one example. Data from soccer can also illuminate one of the most prominent theories of the stock market: the efficient-market hypothesis. This theory posits that the market incorporates information so completely and so quickly that any relevant news is integrated into a stock’s price before anyone has a chance to act on it. This means that unless you have insider information, no stock is a better buy (i.e., undervalued) when compared with any other.
If this theory is correct, the price of an asset should jump up or down when news breaks and then remain perfectly flat until there is more news. But to test this in the real world is difficult. You would need to somehow stop the flow of news while letting trading continue. That seems impossible, since everything that happens in the real world, however boring or uneventful, counts as news.
This is where soccer is useful. In a study published earlier this year in The Economic Journal, the economists Karen Croxson and J. James Reade analyzed live soccer betting markets, looking at second-by-second betting activity around goals scored just seconds before halftime and betting activity during halftime. Their data, which concerned 1,206 Premier League soccer matches in England, contained 160 such “cusp” goals scored within seconds of the end of the first half.
The break in play at halftime provided a golden opportunity to study market efficiency because the playing clock stopped but the betting clock continued. Any drift in halftime betting values would have been evidence against market efficiency, since efficient prices should not drift when there is no news (or goals, in this case). It turned out that when goals arrived within seconds of the end of the first half, betting continued heavily throughout halftime — but the betting values remained constant, a necessary condition to prove that those markets were indeed efficient.
Other research by me and others has shown that data from soccer can shed light on the economics of discrimination, fear, corruption and the dark side of incentives in organizations. In other words, aspects of the beautiful game that are less than beautiful from a fan’s perspective can still be illuminating for economists.
But perhaps most beautiful of all, for me, is that the core principles of my beloved professional discipline are exemplified by my beloved game.


Sunday, June 1, 2014

L'Union européenne comme bouc émissaire

Je pense qu'on souligne insuffisamment combien les hommes politiques (surtout) et les médias (dans une moindre mesure) utilisent l'Union européenne (et surtout la Commission) comme bouc émissaire. On prête notamment à la Commission beaucoup d'arrière-pensées et d'opacité. Cet excellent article du Canard montre ce qu'il en est dans le domaine des négociations internationales.