August 11, 2009

Ahead in the Count

Home-Field Advantages, Part One

by Matt Swartz

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In every sport and at every level, the home team wins more games than the visiting team. While this is true in baseball, it's less the case than in other sports. Throughout baseball history, the home team has won approximately 54 percent of the games played. Nearly every aspect of the game has changed drastically over the last century, but home-field advantage has barely changed at all. Consider the home-field advantage in each decade since 1901:

Decade    Home Win%
1901-09   .533
1910-19   .540
1920-29   .543
1930-39   .553
1940-49   .544
1950-59   .539
1960-69   .540
1970-79   .538
1980-89   .541
1990-99   .535
2000-09   .542

Although small decreases in home-field advantage have occurred at times, any incidence still represented a very small change relative to the large changes in nearly every other baseball statistic over the last century.

I will argue that this should surprise us as analysts more than it does. Nearly every study of psychology with respect to baseball has come up revealing either small effects or no effect. We all know that players are human, but the numbers do not seem to indicate many obvious psychological aspects. Hundreds of researchers have tried to discover clutch hitting, but few have found any evidence of its being a repeatable skill. Researchers have tried to look at synergy effects and if certain players increase a team’s chance of winning, and none have found any. Some have attempted to look for the impact of veterans on playoff races, and have not found significant effects there either. We have attempted all kinds of ways to splice the data to reveal a large psychological effect within baseball to show that baseball players don’t behave like statistical models, and there seems to be little evidence of any strong, detectable effects, even if we know they exist and occasionally can discover smaller ones.

The mantra that we go by as analysts is that talent trumps everything, and that most of the stories of heroism and mental fortitude are narratives written by the winners. However, home-field advantage is perhaps the most obvious area where we see something resembling a psychological effect, or at least an effect that is not captured by our typical models of baseball players and ballgames. It is clear that something about being the home team trumps talent in a way that is mathematically equivalent to benching an average player on the road team.

The reason why home teams have an advantage is less clear. Do they feel more at home and thus perform better? Does the crowd excite the home team or distract the road team? Do they know how the stadium plays better? Are they simply more comfortable when at home? All of these explanations seem possible, and we could give a number of plausible explanations. Finding out why the home team wins more often could open the doors to measuring at least one area where psychology has an impact in baseball. Everybody knows that baseball players are not computers, let alone Strat cards. The mainstream media certainly dramatizes the purported impact of psychological effects on players, but there must clearly be some effect if the home team wins more often.

This article will be the first part of a series of articles discussing home-field advantage. This series will ask what, when, where, when, and how, and hopefully will figure out why home teams win more often. The first step is to figure out what exactly it is that home teams are doing better. If we can look at which statistics show the biggest home-field advantages, we can figure out what exactly the home teams are doing to their opponents to consistently win more often than they lose.

Innings

Home-field advantage may come from the effects of comfort and familiarity, or it may come from the psychological effect of the crowd support. If the effects primarily come from comfort and familiarity, one would expect that home teams would have an even more distinct advantage in early innings. If the effects are primarily psychological, one would expect that home teams would have an even more distinct advantage in late innings when the crowd may put them on top.

The percent difference in runs scored by the home team as compared with the away team by inning is listed below. I ignored the ninth and any extra innings, because the home team stops scoring when they take the lead in extra innings or in the ninth inning, and the home team frequently does not bat in the ninth inning at all.

Inning  % of More
        Runs at Home
  1      16.2%
  2       9.3%
  3      10.1%
  4       6.0%
  5       7.8%
  6       8.1%
  7       8.7%
  8       6.5%

The home teams are gaining a lot of their advantage in the first inning, and enjoy a greater home-field advantage in the second and third innings than in the fourth through eighth innings. This implies that home-field advantage is primarily about comfort and adjusting to surroundings. Consider this as we discuss individual statistics below.

The Four True Outcomes

There are multiple people who play a role in the outcome of a plate appearance. First, the pitcher throws to the batter, and the batter either does or does not swing. If he does swing, he may or may not hit the ball. If he hits the ball, it may or may not be a fair ball in the field of play. It has been well established that pitchers have little control over what happens when the ball is in the field of play—pitchers primarily control walks, strikeouts, and home runs, and the rest is mostly a product of the defense and the hitter. In this section, I will discuss what happens when the ball is not in the field of play to see how much of home-field advantage is the responsibility of pitching. All data is from cumulative major league totals from 1998-2008, unless specified otherwise.

Strikeouts and Walks: Batters struck out in 16.35 percent of their plate appearances at home from 1998-2008, and in 17.30 percent of their plate appearances on the road. Batters received unintentional walks during 8.38 percent of their plate appearances while at home, and 7.80 percent of their plate appearances while on the road. Both of these numbers are statistically significant at the 99.9 percent level. This immediately indicates that a home-field advantage occurs within the strike zone. Somewhere between when the pitcher starts his windup and when the batter swings, home teams already gain an advantage over road teams.

Home Runs: Batters hit home runs in 3.22 percent of their at-bats at home, and in just 3.07 percent of their at-bats on the road. This is also significant at the 99.9 percent level. Home teams are able to hit for more power than road teams.

I selected strikeouts, walks, and home runs as the initial tests, because they indicate that the pitcher is involved. The significance of home runs, something that pitchers control less than walks and strikeouts, indicates the hitter must be doing something better at home as well. However, without any more information, it is difficult to tell if the walks and strikeouts are only the products of home effects on hitters. For instance, it may be that given enough time to prepare himself to pitch, the pitcher can overcome any disadvantage of being on the road, but the hitter has an advantage at reacting to the baseball when at home. In other words, it’s possible that travel or staying in a hotel dulls hitters’ reaction times, but do not hamper the pitcher. That’s why I now introduce the fourth true outcome—the neglected one on which fielders also have no effect.

Hit By Pitches: In every 1000 plate appearances, home hitters were hit by 9.41 pitches and road hitters were hit by 9.09 pitches. This is a statistically significant difference (p=.016). While some hitters may try to dodge pitches out of the strike zone more often, very few players can actually lean into the ball better. The fact that hitters at home are hit more than hitters on the road indicates that some of the home-field advantage is definitely on the pitcher. Getting hit by pitches is a persistent skill for batters, but it primarily derives from hugging the plate and not dodging inside pitches. Unless hitters hug the plate much more at home—and that this effect somehow dwarfs the extra reaction time at home to dodge pitches—chances are that this effect comes down to pitchers’ aim being worse on the road. While some of this may be umpiring making close calls as hit by pitches or more consistently awarding the base to a home batsman instead of ruling he made no effort to avoid the pitch, the 3.5 percent increase in hit by pitches is probably representative of a real, home effect-generated difference.

Balls in Play

When a ball is hit into play, defense becomes a factor. Some success on balls in play is the effect of how well the hitter hit the ball, but much of it is a product of how the defense plays as well. Looking at balls in play will be the first clue as to how defense plays a role home-field advantage.

Batting Average on Balls in Play: Home teams had a .301 BABIP, and road teams had a .295 BABIP during 1998-2008. Furthermore, when you factor in errors, home teams reach base a .316 clip on balls in play (or fail to reach base on 68.4 percent of balls in play), and road teams do so at a .310 clip (and thus fail to reach on 69.0 percent of balls in play). When you factor in double plays, defenses at home are able to record .720 outs per ball in play, and defenses on the road are only able to record .713 outs per ball in play. All of these differences are statistically significant at the 99.9 percent level. It is clear that batters are able to reach safely at home more often. It is not clear how much of the difference in outs on balls in play is hitting and how much is fielding, so it is useful to look at some indicators of defense to provide some clues.

Double Plays: Hitters ground into double plays in 2.92 percent of their balls in play at home, and 2.94% of their balls in play on the road. This is not a statistically significant difference. Initially, that sounds like the difference is hitting, until you realize that home teams have higher OBP than road teams (.342 vs. .330), so they have more opportunities to bat with a man on first base. When I approximate opportunities by the number of singles, walks, hit by pitches, and times reached on error that home and away teams have, the difference suddenly becomes statistically significant. The ratio of double plays to the sum of singles, walks, hit by pitches, and reach on errors at home is 7.62 percent, and the ratio on the road is 7.96 percent. Although some of this may be the hitters’ fault for swinging at bad pitches or making weak contact, this does seem to imply that defense is better at home. As some of this may be umpires calling away-team baserunners out on double plays more often, it's useful to check on other aspects of fielding too.

Reaching on Errors: Official scorers are often accused of giving the home team credit for hits more often, while recording road players’ close calls as errors. This may or may not be true, but it is highly doubtful that the reverse is true. Therefore, the fact that home teams reach on error during 1.46 percent of their balls in play and road teams do on 1.42 percent of their balls in play implies that players on the road are fielding more poorly. The ratio of infield hits to the sum of infield hits and times on reached on error is 70.3 percent for home teams and 69.8 percent for road teams. This is also not a statistically significant difference, which further suggests that official scorers are probably not all that biased. To me, this suggests that chances are that defenses are worse on the road.

Doubles and Triples: Another potential source of home-field advantage is that outfielders at home know their parks better—they know how the ball bounces off the wall, etc. Many triples are the result of balls that get away from outfielders. Therefore, it should not be surprising that the rate of triples per ball in play is 7.59 percent for home teams and only 6.44 percent for road teams. That is a huge and statistically significant difference. In fact, the rate of doubles on balls in play for home teams is 6.81 percent, and for away teams is 6.74 percent, which is not even a statistically significant difference (though it is weakly significant). The reason is that so many extra-base hits get away from outfielders and turn into triples; the ratio of doubles to triples is 9.0 at home, and 10.5 on the road.

Shutouts

Pitchers pitch complete=game shutouts far more often at home than on the road. Per every 100 games started, the hometown hurler will throw 1.68 complete-game shutouts and the road team hurler will throw 1.18. Even though home teams only win 54 percent of games, home teams record 59 percent of the complete-game shutouts.

Unearned Runs

It may seem like much of this could still be psychological, but I believe that a large portion of it is not. Some of the reasons for that will be clearer in subsequent articles in this series, but one indicator will be the look at unearned runs. If psychology is playing a significant role, then chances are that it will cause pitchers to be more prone to frustration and let problems escalate. One way to get some insight about this is to check the ratio of unearned runs to the times that hitters reach base on errors. If pitchers are less likely to be collected on the road, they will probably let more of these runners score. In fact, the ratio for home teams is 0.98 and 0.99 for road teams. This is a very small difference. In fact, from 1998-2008, the home team had a higher ratio than the road team six times out of eleven years. It does not seem like psychology is playing a role—road pitchers are not allowing crises to escalate any more; they are just finding themselves in more crises.

Stolen-Base Percentage

Home teams are more successful in their stolen-base attempts, nabbing bags successfully in 71.2 percent of their attempts, while road teams are successful in 69.4 percent of theirs. This is a strongly statistically significant difference. Whether that is due to catchers reacting slower, baserunners running faster, or umpires giving close calls the home crowd is tough to know. For now, it seems that there is a clear tendency to steal successfully while at home.

Batted Balls

A little bit about home-field advantage can be inferred from batted-ball statistics. Looking at data from 2005-2008, we can see that home teams hit more line drives (19.0 vs. 18.6 percent) and fewer ground balls (45.1 vs. 45.5 percent) than away teams; they hit about the same number of fly balls. The differences in line drives and ground balls are both statistically significant. As ground-ball rate is something that pitchers control strongly, this also suggests not simply that pitchers enjoy this effect of home-field advantage, but that they affect it. The batter also plays a large role as well; this is clear from the statistically significant difference in popups per fly ball at home vs. away (21.8 vs. 22.2 percent), as infield-fly rate is something that batters affect more than pitchers do.

Hitters do a little bit better on each type of batted ball as well. Look at the slash stats for each type of batted ball:

Batted Ball Type        Home              Away
Ground balls       .248/.248/ .270   .239/.239/.259
Fly balls          .224/.218/ .597   .218/.213/.578
Line Drives        .733/.731/1.105   .732/.729/.997

It is quite clear that home-field advantage affects each type of batted ball. It does not seem to affect the batting average on line drives very much, but it certainly seems to affect the ISO on them. There are more triples than doubles, and there are a good deal more home runs as well (2.5 vs. 2.0 percent). The home-run rate on fly balls is also a bit higher in for home teams (9.2 vs. 8.9 percent), and the rate of triples on fly balls is higher too (1.13 vs. 0.95 percent). Ground balls find their way through holes more, leading to more singles (22.6 percent vs. 21.9 percent of ground balls), more doubles (2.02 percent vs. 1.89 percent), and more triples (.112 percent vs. .086 percent). The relatively larger jumps in doubles and triples on ground balls indicate that a lot of home-field advantage is knowing how to play the bounces in one’s home stadium.

Putting Things Together and Moving Forward

It appears that home teams do pretty much everything better than road teams. They hit the ball more, they hit the ball harder, they throw more strikeouts, and they surrender fewer walks. Home teams prevent triples more frequently, and record double plays more frequently. They record more outs on balls in play, and they make fewer errors.

The extra home-field advantage in the first three innings strongly suggests comfort and familiarity are significant factors in home-field advantage. The large effects within the strike zone indicate that the mound and the batter’s eye are things that pitchers and hitters are more comfortable with at home. The extra triples surrendered on the road strongly suggest an impact of stadium familiarity in home-field advantage. The extra stolen bases indicate reaction time may also be playing a role. All of these numbers together indicate a large significance of mental aspects that are not quite emotional, but affect things like eyesight, reaction time, and learning about the home turf. That pretty much answers the question of what home teams are doing better.

Next week, I will begin to answer the question of who has a home field advantage, and I think that the answer may surprise you. Now that we know what home teams are doing better, our next stop is to check differences between teams in terms of how much better they perform at home. After that, we will begin to discuss the where, when, and even broach the questions of how and why home-field advantage exists.

Matt Swartz is an author of Baseball Prospectus. 
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