Tuesday, July 19, 2011

Home Court Advantage

It is time for some light summer reading and David Runciman is ready to oblige. The occasion is a review he has written of the book:

Scorecasting: The Hidden Influences behind How Sports Are Played and Games Are Won by Tobias Moskowitz and Jon Wertheim
Crown, 278 pp, £19.50, January 2011, ISBN 978 0 307 59179 1

The review can be found in the London Review of Books under the title: Swing for the Fences. Runciman summarizes his thoughts on the book tersely.

“It is by far the most engaging book of its kind yet published, crisply written, extensively researched and full of surprises. The biggest surprise of all is home advantage.”

It is the single subject of what Runciman calls “home advantage” that is the focus of the article.

“Add up the total number of home victories and compare it to the total number of away victories. The ratio will be at least 60:40 in favour of the home sides (often it’s more: in the English Premier League home advantage currently runs at around 63 per cent, in Spain’s La Liga it’s 65 and Italy’s Serie A it’s 67). The advantage holds across almost every major sport, though exactly how big it is tends to vary. Fans are so used to this that they take it for granted their team is much more likely to win on its own turf. They also take it for granted that they know why – it’s because the home crowd is cheering the team on. But there is no evidence for this.”

Runciman summarizes the explanation Moskowitz and Wertheim produce as if it is the inevitable answer, and then proceeds to disagree and provide one of his own.

Moskowitz and Wertheim evaluate the statistics available on individual performance with home and away being the only variables. They look at things like free throw shooting, field goal kicking, and penalty kicks and find no differences between home and away. They look at the effects of travel and the peculiarities of given home fields and again find no explanation. They arrive at the conclusion that the home advantage must derive from the referees and umpires.

“Moskowitz and Wertheim find plenty of evidence to back this up. In football, it turns out that referees consistently award more injury time when home teams are losing, and less when they are winning (on average, four minutes in the first case and two minutes in the second, enough to make a difference in plenty of matches). Home teams get far fewer players sent off, and receive many more free-kicks. Maybe this is down to the fact that the home side simply plays better and the away players are reduced to desperate measures. But Moskowitz and Wertheim find evidence that crowd effects make a real difference. In the German Bundesliga, for instance, where many of the teams used to play in stadiums incorporating running tracks, putting the crowd much further away from the action, the bias referees normally show to the home side was cut in half. In the British, Spanish and Italian leagues, attendance also has a marked effect on the number of red cards shown to the visitors. The bigger the crowd, the more likely the away team are to end up with fewer players on the pitch at the end.”

“However, the most compelling evidence for referee bias comes from those sports that have introduced technology to check on the decision-making of the officials. In baseball, a system called QuesTec (similar to Hawk-Eye in cricket and tennis) now shows whether a pitch was in the strike zone or not (the area over the home plate between a batter’s armpits and his knees). Moskowitz and Wertheim have looked at a mass of data and discovered that when a pitch is clearly a strike, baseball umpires do not advantage the home hitters. Equally, when a pitch is way outside the strike zone, they call it against the pitcher. But when it’s on the edges, the home team were getting a large percentage of favourable calls. This shows two things. First, given the choice, umpires prefer to please the locals who are breathing down their necks (in many baseball stadiums almost literally). Second, they know what they are doing – they restrict their bias to areas where it won’t be so obvious (in stadiums that have installed QuesTec umpires have started to eliminate their home bias, now that they realise it’s there for all to see). Moskowitz and Wertheim find the same thing in ice hockey and American football, where the introduction of instant replay reviews showed that for close calls, and in tight games, the officials tend to favour the home team by a significant margin (calls against the away side are more likely to be corrected when impartial technology is called in evidence). Tight games are by definition the ones that can turn on one or two key decisions. And it appears that tight games are also the ones in which the officials go out of their way to help the home team. That’s enough for Wertheim and Moskowitz to finger them as almost entirely responsible for the phenomenon of home advantage.”

Runciman grants that the referees and umpires do contribute to home court advantage, but denies that it is possible to explain everything with that one factor. He tells us of a Portuguese football coach who amassed an amazing home record of 150 straight victories over more than nine years and with four different clubs in four different countries. He estimates that the odds that this could happen are more than seven million to one. The implication is that this one coach seems to have understood what makes the home advantage work, and it is not as simple as working the referees and umpires.

Runciman finds the major error with the authors’ analysis is that they have focused on individual performance rather than team performance.

“Moskowitz and Wertheim claim that the performance of players from the free-throw line or the penalty spot shows the crowd doesn’t have an impact on the performance of the home team. But that’s not what it shows at all: it shows that the crowd doesn’t have an impact on individuals. What if home advantage is a team phenomenon? There is plenty of evidence not considered by Moskowitz and Wertheim to suggest that it is.”

“Somehow, playing at home breeds a sense of solidarity, or what used to be called team spirit, which means that players have more confidence in each other and work better as a unit. I’m not saying that’s definitely what happens. But Moskowitz and Wertheim haven’t proved that it doesn’t.”

He then uses the differences in home field advantage between baseball and football (soccer) to illustrate his point. Baseball is played by teams, but it consists mainly of individuals performing alone, whereas football is clearly a concerted team effort.

“Baseball has a relatively low home advantage ratio – the lowest for all major sports – at around 54 per cent for the major leagues. This is a huge difference from the 63-67 per cent that holds for the big European soccer leagues. What explains it? Moskowitz and Wertheim spend a lot of time describing how the bias of baseball umpires can account for almost all the home advantage in that sport – if it sways around 3 per cent of games (and they give good reasons for thinking that it does), then that’s practically the whole of it. But what about the extra 10 per cent in football?”

Runciman contends that the authors do not have a valid explanation for the football home advantage based on their referee hypothesis. In fact he uses their analysis to argue against them.

“As well as identifying a bias in favour of the home side, they also show that officials prefer to avoid making decisions that might make them stand out, especially near the end of a game. This is a widespread phenomenon, and it applies to football as much as any other sport. As Moskowitz and Wertheim report, having studied 15 years of data from the Premier League, La Liga and Serie A: ‘Fouls, offsides and free kicks diminish significantly as close matches draw to a close.’ This is the ‘omission’ bias, and we all tend to suffer from it – we prefer to let bad things happen than to take a chance on doing the right thing and risk carrying the can. If a referee intervenes in a game near the end, it looks like he’s deciding the outcome. That’s going to make some people mad.”

It is hard to blame referees when the referees are trying as hard as they can to not be involved at the end of a tight match.

Runciman leaves us with the argument that home advantage derives from a complex mixture of factors, one of which is refereeing, that allow the home team to play with more confidence and more effectiveness. He then tries to gain some insight from the unlikely success of Mourinho.

Runciman introduces one more factor that he refers to as “loss aversion.”

“In any team sport, players and managers suffer from omission bias too: they don’t want to make the dumb mistake or dumb tactical switch that means they can be fingered for a loss. Closely related to omission bias is loss aversion, which is also a widespread human trait. People prefer avoiding losses to acquiring gains....most coaches would rather avoid contributing to a loss than take a chance on contributing to a win: they play it safe. Across all sports, coaches tend to make the plays that give them the best chance of preserving what they have (including their jobs). But it also means that the best coaches recognise this weakness in their rivals and exploit it. Widespread loss aversion gives the coaches who don’t mind looking stupid a competitive advantage.”

“The most plausible explanation for his [Mourinho’s] astonishing home record is that he is a manager who is not afraid to take risks. He has a reputation as a conservative coach, but in fact he is very adventurous, albeit in an unattractive way. He’s not afraid of doing ugly things that draw attention to himself – he actually seems to relish it.”

Runciman goes a little mysterious at this point, because he doesn’t feel that anything that has been discussed can fully explain Mourinho and his success.

“One of the many ugly aspects to the way Mourinho’s teams play is that they are not afraid to harass the referees. No doubt Mourinho worked out early on that this is an important part of home advantage. But that’s not all he’s worked out. His teams are so hard to beat at home for reasons that go well beyond the reductive account provided by Moskowitz and Wertheim. Home advantage is much more complicated and much more mysterious. It depends on a range of factors that are effectively impossible to quantify. Maybe even Mourinho’s gorgeous clothes are part of the package: perhaps looking good helps to instil confidence in his team that their boss is the boss, and his territory is theirs to defend. But whatever is really going on, Mourinho must know that the idea home advantage can be reduced to, and blamed on, referees is just the sort of conservative, risk-averse thinking that gets you into trouble. So he knows something the freakonomists don’t: you can’t always trust the numbers.”

My inexpert, qualitative analysis of competitive sports over the years leads me to agree with Runciman that the home court or field does something to the home team that allows them to play better as a unit. Call it confidence, or adrenaline derived from the excitement from the crowd, or whatever. I believe it is a real factor. We will all have something new to ponder as we watch our favorite team win at home and go belly up on the road. As for Mourinho, perhaps he knows something other coaches don’t—or perhaps he is just another highly improbable event.

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