Henry subscribed me to Sports Illustrated. I’m thankful, but it’s also a curse, since that means yet another magazine I must read cover to cover every week.

That said, I really enjoyed last week’s issue. I realize I write about sports a lot, far more than it’s actually on my mind, but this issue had an article on home field advantage that was one of the most interesting things I’ve ever read. I can’t find a link online, so let me summarize it here.

So home field advantage is a real phenomenon – in pretty much every sport, the home team wins more frequently than the away team. The question is,
what is the source of home field advantage?

First, some amazing facts about home field advantage. Within a sport, the advantage (percentage of games won by the home team) is incredibly consistent, across time (it’s the same as 50 or 100 years ago) and across different countries and leagues. The home field advantage in Arena Football is about the same as in the NFL. The advantage in the NBA is the same as in the WNBA. In 42 soccer leagues across the world, the rate of games won by home team is virtually the same, between 63% and 65%. Amazing.

There are some common theories as to why home teams win more often. The article examines them, and finds that most aren’t true.

Home teams win because their crowds boost players’ performance. There are certain situations where one can measure the direct effect of the crowd on a single player’s performance, for example looking at free throws in basketball. If the crowd affects basketball players’ performance, one would expect the home team to make their free throws at a higher rate than the away team. And over the last 20 years, visiting teams in the NBA made their free throws 75.9% of the time. Home teams: 75.9%. It doesn’t matter if you look at free throws in specific quarters or overtime, or by how close the game is (when the crowd might be more into the game and have a greater effect); home and away teams make their free throws at exactly the same rate.

Hockey has a similar situation with shootouts, and we see the same thing – home and away teams each win about half of all shootouts. In football, home and visiting punters kick exactly as far (41.5 yards), and field goal accuracy for kickers is exactly the same whether home or away (72%). In baseball, for the past few years MLB has been tracking every pitch with Pitch f/x, and home and away pitchers hit the strike zone at the exact same rate, with the same velocity and same movement. In short, the crowd seems to make no difference in player performance at all.

Home teams win because the rigors of travel doom visitors. Meaning, visiting teams are more tired, so they perform worse. Turns out, this isn’t true. We know this because the home court advantage is exactly the same even when the “away” team plays in the same arena (e.g. with the Lakers and Clippers) or doesn’t have to travel far (e.g. Giants/A’s, Cubs/White Sox). Nor is the home court advantage greater the farther the team travels. Furthermore, the home court advantage within a sport is exactly the same as it was decades ago, when the mode of travel was more onerous, for example when they traveled on trains vs. the planes they fly on now.

Home teams win because they benefit from a kinder, gentler schedule. This one is somewhat true. In the NBA, almost all back-to-back games are played by visiting teams, and they win only 36% of the time. Home teams also get more off days. The article estimates that 21% of the NBA’s home team advantage is due to scheduling. However, it seems to have no effect on baseball or football. Scheduling does effect college football where some of the home field advantage is due to big conference schools scheduling easy teams at home early in the season. If one looks only at conference games, the home winning percentage in college football goes from 64% to 57%. Amazingly, the home team’s 57% winning percentage is about the same as in the NFL or Arena Football.

Home teams benefit from unique “home” characteristics. Broadcasters claim this all the time, that cold weather teams do better at home because warm weather teams don’t know how to play there. Also turns out to be untrue. Based on NFL games from 1985 to 2009, cold weather teams don’t win any more at home in cold weather, nor do warm weather teams win more at home in hot weather. Even in baseball, which has different field dimensions that teams might build for, there’s no difference in hitting or pitching away vs. home.

So what drives home field advantage? It turns out that it’s almost entirely driven by officials’ bias.

One study examined 750 matches in a Spanish soccer league, and found that refs gave more extra time when the home team was behind at the end of regulation (4 minutes), and less when the home team was ahead (2 minutes). In a tie game, they gave 3 minute of extra time on average. But if the home team was significantly ahead or behind (two or more goals) and the extra time had little chance of affecting the outcome, there was no bias at all. The same is found in leagues in Germany, England, the MLS, and many other leagues. The home team also receives far fewer red and yellow cards. Add these up, and it makes sense why the home team in soccer wins 63% of the time.

In baseball, it turns out that the main difference between how home and away teams perform is that home teams strike out less and walk more – a lot more. We’ve already seen (based on Pitch f/x) that pitchers don’t actually throw better at home. When looking at only called balls and strikes (not swings / fouled off balls or intentional walks), home batter get far fewer strikes per called pitch than away batters. In fact, when examining Pitch f/x, it turns out that pitches in the exact same location are called differently for home vs. away batters. Intriguingly, the more critical the game situation, the greater the difference in called strikes for the home batters. Stolen bases and double plays, also an umpire judgment call, are also called in favor of the home team more often.

Based on the authors’ analysis, the way umpires call balls and strikes differently for home vs. away teams accounts for 7.3 runs a season. Since home teams outscore visitors about 10.5 runs per season, umpires’ bias in calling pitches for the home team accounts for 2/3 of the home field advantage in baseball.

In football, home teams receive fewer penalties than away teams. As with soccer and baseball, the more crucial the situation, the higher the bias for the home team. More evidence of referee bias is shown by instant replay challenges, which started in 1999 and allow teams to reverse bad calls. Before challenges, home teams had an 8% advantage in turnovers. After replay challenges, that advantage was cut in half. Before replay, the home team lost fewer fumbles than the away team. After, that difference disappeared. In close games, the home team’s fumble recovery rate used to be even higher, 12%. After replay, it also disappeared. When looking at penalties in games where the home team is behind, the home teams’ challenges are successful 28.4% of the time. The away teams’ challenges are successful 40.0% of the time, suggesting they get more bad calls.

So what’s going on? It’s not that referees are intentionally favoring the home teams. It’s that, like all people, they’re subliminally influenced by the crowd. The greater the crowd influence, the more likely the referees are to be affected. This is shown in the data – in countries (like Germany) where the crowds are further away from the field (and thus exert less influence), the refs bias is less. Umpires’ bias in calling strikes/balls is also less pronounced when the attendance is low. Traveling is called more often against the away team when attendance is higher. In almost every sport, the crowd size significantly affects the home field advantage, and it’s reflected in how the referees call the games.

This also explains why the rate of home field advantage in a sport tends to stay the same over time. The rules change, but the role of the referee in the game, and thus their effect on it, doesn’t. If that role did change, e.g. if baseball moved to using computers to call balls and strikes, we would almost certainly see home field advantage in baseball go down.

So home field advantage is mostly caused by referees bias in ways that favor the home team. Fascinating.

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