Home-field advantage isn’t always considered a matter of great importance in baseball. Crowds aren’t as close to the action as they are in basketball. There’s nothing comparable in the sport to something like raucous Cameron Indoor Stadium. There are no 100,000-seat, canyon-like stadiums cascading noise to the playing surface like in college football.

But home-field advantage is a real thing in baseball, and significant, and has remained constant for better than a century.

The road winning percentage of visiting teams was .461 in the 1910s. Road winning percentage stands at .464 to date in the 2010s. Road winning percentage has remained consistent over the decades.

Since the 1970s…

Home-Field Advantage Is Stable Decade Road winning % 2010s 0.464 2000s 0.456 1990s 0.464 1980s 0.460 1970s 0.463

Conventional wisdom has it that home-field advantage is derived from some combination of hostile crowds, fatigue from travel, and the familiarity of the playing surface. And, in a way that’s unique to baseball, teams can tailor their roster to their home park’s dimensions. Having carried on that way for better than a century, the home-field edge seems to be something of a scientific law.

Home-field advantage is important. It’s a carrot to chase in the regular season, and is often a crucial advantage in the postseason. Sports on Earth found that there have been 154 best-of-seven series in MLB history following “the current 2-3-2 home-field pattern.” The team with home-field advantage has won 85 of those series, good for a .552 winning percentage.

But what if some other force is driving home-field advantage and it’s in danger? What if some savvy, innovative teams can close the margin that exists between home and road teams? What if playing fields tilt closer to neutral?

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University of Chicago behavioral economist Tobias Moskowitz and Sports Illustrated writer L. Jon Wertheim argued convincingly in their book Scorecasting – published in 2011 – that home-field advantage is largely tied to umpire bias, and referee bias in other sports.

In what was innovative research at the time, the authors analyzed several years of PITCHf/x and QuesTec pitch-location data to show that home teams typically received the benefit of borderline pitch calls, particularly in higher-leverage situations.

Wrote Moskowitz and Wertheim:

“In baseball it turns out that the most significant difference between home and away teams is that the home teams strike out less and walk more — a lot more — per plate appearance than road teams.”

And in 2016, pitchers continued to strike out more, and walk fewer, when at home.

Evidence of Umpire Bias? Road/Home K/9 BB/9 FIP NL Road 8.1 3.4 4.27 AL Road 7.9 3.2 4.35 NL Home 8.3 3.1 4.00 AL Home 8.1 2.9 4.15

The authors calculated the advantage equated to 7.3 runs per season for each home team, while noting home teams on average outscore visitors by 10.5 runs per season. If accurate, that bias accounts for roughly 70 percent of home-field advantage.

Anecdotally, more borderline calls seem to go against visiting hitters than home batters.

To see if this bias continues to be a significant factor and to see if it has weakened or strengthened since the Scorecasting findings, I asked FanGraphs’ own Jonah Pemstein to research the home/road splits of all balls and called strikes from 2008 to -16.

Here is a look at all called strikes for home teams 2008-15:

All called strikes for road teams 2008-15:

Compare that to called strikes for home teams 2016:

And road teams in 2016:

The force still appears to be quite strong, and it could tied to a conscious or subconscious desire to comform to the crowd. The Scorecasting authors noted several psychological studies that concluded humans conform because they want to fit into the group and because they believe the group is better informed.

Whatever the reason, the bias appears to exist after nine seasons of PITCHf/x tracking every pitch in every major-league park. The system was designed in large part to improve umpire accuracy.

After each game called behind the plate, umpires are given a report with their calls and the corresponding pitch-location data, along with accompanying video. While the strike zone is evolving — upon which subject The Hardball Times has published some excellent research — and while there’s some evidence of improvement among umpires in the PITCHf/x era, the bias in favor of the home team appears to remain remarkably consistent despite year-to-year changes in strike-zone size and umpire performance.

The following graphs are the most powerful data we identified.

Consider the rates of out-of-zone strikes for home versus road teams from 2008 to -16:

And then the rates of in-zone balls from 2008 to -16:

So that leads us to this interesting possibility: Major League Baseball could perhaps eliminate much of home-field advantage by simply handing over ball/strike duties to robot umps. (And it seems there is a greater call for that with each passing postseason.)

If 70% of home-field advantage is tied to umpire bias, then much of home-field advantage could be eliminated fairly easily.

MLB commissioner Rob Manfred told USA Today in October that pitch-tracking technology is impressive but he does not believe the game is ready for robot umps… yet.

From that piece:

“The technology of calling balls and strikes without a human being involved has continued to improve,” Manfred said. “The principal reason we’ve always done it after the fact is that unlike the box you see on a broadcast, [for] our system that we use to grade our umpires, someone goes in and manually adjusts the strike zone for the batter. As technology continues to improve and those sorts of adjustments can be made [in] real time, that technology will become more feasible for use on the field. I don’t believe we are there yet.”

If the technology isn’t there yet, it’s close. Perhaps all that’s required is for a personalized height of zone to be established for all players from Jose Altuve to Giancarlo Stanton, for each player to be attached to sensors each spring and have their strike zone measured.

Yet, even if you believe bias explains much of home-field advantage, it doesn’t explain the totality of it. And perhaps advances are being made in other areas relevant to the phenomenon.

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Will Ahmed might be on to something.

As a squash player at Harvard he was around athletes, he said, “who didn’t really know what they were doing to their bodies.”

Ahmed felt he and his teammates “misinterpreted fitness peaks.” He felt there wasn’t enough understanding of the importance of recovery and rest.

That was the spark for his startup company, Whoop, which is part of the wearable technology wave that has made its way to professional sports and Major League Baseball.

Ahmed says that, last year, he tested his product with 230 minor-league players in a partnership with MLB. Whoop is worn around the wrist like your FitBit but it’s different. Ahmed says it does not measure “steps” but rather “strain.” The device monitors heart rate, heart-rate variability, ambient temperature and perspiration.

While the technology is designed to better enhance an athlete’s efficiency, it was studying the rest and recovery of minor-league players on the road led Ahmed to a “eureka” moment.

“We found that athletes, when traveling, on average got an hour less sleep than athletes that did not travel,” Ahmed said. “An hour less in bed, 45 minutes less sleep. That’s about 10% lower recovery. Conventional wisdom says the home team wins because they have their fans supporting them or they are playing in a familiar park. The study suggests the home team may win because they are better recovered and better well rested.”

Ahmed found the minor leaguers he was studying were slightly less effective on the the road after traveling, and in returning home, because they were less likely to be at full recovery.

“There was a direct correlation between the higher your recovery and the faster your exit [batted ball] velocity. We found the same correlation with pitchers and fastballs,” Ahmed said. “I didn’t know for sure we were going to see such strong correlations with travel, recovery and performance. Those were a bit of a eureka [moment] with us and for people in Major League Baseball who saw this study.”

Ahmed said that among the 28 teams that experimented with Whoop, 70% of players participated. Whoop gives a metric before 0% and 100% in regard to recovery. He said habits changed.

“The athletes themselves are self-managing,” Ahmed said. “One thing we found is athletes would talk about drinking less alcohol or going to bed earlier. They are starting to think about their recovery. The higher their recovery the better they play. You go from thinking of yourself as an athlete three hours a day, to thinking of yourself as an athlete 24/7.”

In Scorecasting, the authors discounted the travel fatigue as a reason behind home-field advantage.

“[Home—field advantage] was virtually the same win MLB from 1903 to 1909 as it was from 2003 to 2009. This suggests that teams jetting on chartered flights with catered meals, high-thread count linens and flat-screen televisions have no more success than did teams that traveled to games in Pullmans and buses. (Either travel isn’t causing the home advantage or teams need to rethink their jet purchases and on-flight catering).”

To their point, no team travels more miles than the Seattle Mariners in a given season given their northwest location. While the Mariners have a lowly .435 road winning percentage since their inception – below the MLB rate of 46.2% since 1970 – the Mariners’ home-road winning percentage differential of 6.8 points is below that of the MLB average 7.7 since 1970.

But as more and more teams and players become more conscious of rest and recovery, as players learn how to better travel, perhaps the old data becomes less relevant.

Teams and players know more about performance and rest today than they ever have. Many NBA, NFL, NHL and MLB teams are trying to better understand how to improve player performance through rest and recovery.

Some major-league teams installed recovery rooms in their clubhouses, players have met with sleep experts and have been advised to travel with eye masks and their own pillows. In the NBA, home teams are perhaps losing some of their home-court edge. NBA teams are more and more curious about how best to manage travel and deal with a demanding schedule. In baseball, perhaps these changes and adoptions will also erode some of the home team’s century-plus edge.

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There is another question to be answered: do we want to rid the game of home-field advantage?

While baseball’s home-field isn’t as powerful as those in basketball and football, it is significant. It creates regular-season incentive and reward. So perhaps those pining for robot umps should pause and think about all of those implications. Do we want all fields to play neutral or closer to neutral?

Perhaps no matter how many advances and adoptions in technology there are, the home team will always have a significant edge. It’s been something of a constant for more than 100 years. Or perhaps the current gap between home and road teams is more fragile than we think.