Some possibilities that have nothing to do with psychology:

1. GM's select players best able to take advantage of their home environment.

2. The effect of twilight and shadows on the pitcher/batter confrontation. (The effect should lessen as the game goes on with the advantage going to the team hitting in the bottom of every inning.)

3. Perhaps I missed this being discussed....but obviously home teams win more often because they have the strategic advantage of hitting last.

It would be interesting to see how well this effect holds up during the 2nd and 3rd games vs. the first game of an away series, to determine whether the road team is able to adjust. Although I wouldn't want to try to compile that data set...

Interesting stuff. A couple of questions that may be jumping the gun on the future columns.

1. The strikeout/walk data make me wonder if the home crowd may have a psychological impact on the umps. Do umps favor the home team, ever so slightly, in defining the strike zone? Could this then have a follow-on effect on all the batting data?

2. Travel wearininess. Not a psychological or stadium-comfort issue, but a real possibility for inducing a small but across the board decline in player effectiveness. I, for one, sleep like crap away from my own bed, not that I'm an athlete.

3. The big one. How much of the difference in win rate for home teams is simply the result of the advantage of batting last? Given two identical teams, the home team will win a certain amount more simply because they have more margin for error in late and extra innings. Gave up a run in the top of the tenth? Score two in the bottom and you're still good. Give up that solo dinger in the bottom of the frame, walk off. Run a playoff odds report-style million game simulation between two identical teams and see how often the home team wins. If home teams win 53% of the time just because of that factor, and 54% of the time overall, all the stats are only accounting for a tiny part of the advantage.

These sound very similar to the numbers one would expect if the home team got to play with a slightly smaller strike zone when batting as compared to the road team. This was an excellent article and I look forward to the rest in the series.

I have always wondered if the strength of home field advantage is correlated with attendance...does a packed stadium give more of an edge to the home team than an empty one does in a measurable way?

Sounds like that may be coming in future installations, which I really look forward to!

if comfort and familarity are an advantage for home teams, would the visiting teams performance improve in the 2nd or 3rd game of a series, or in later series in the same season?

What a great article.

With regard to comfort and familiarity as a component of the home-field advantage, is it possible to compare data from the first game a series to the subsequent games in a series? The advantage seen in the first three innings may be less in subsequent games. Along the same lines, what is the home winning percentage for the first game of a series compared to subsequent games in a series? If familiarity is a key component, you might find that home winning percentage is highest for game 1 and lower for the subsequent games.

Keep the series coming!

Great stuff Matt. One thing to keep in mind: the HFA is actually much smaller in baseball than it is in the NBA or NFL, even though both are played on homogeneous surfaces. Both are anecdotally more momentum/crowd/emotion driven than baseball, and it's funny to think that pure psychology (which we tend to crap on) could have such a much bigger impact than actual tangible differences in the teams' home ballparks.

Great stuff. To me, a lot of the advantage can be traced to the pitcher's mound. Listening to pitchers, each mound is unique (whether it is pitch, the kind of dirt, whatever) and also differs from the bullpen mounds on which the starters warm-up. It seems reasonable that the difficulty in locating and commanding pitches stems from the visiting team's starter being unfamiliar and/or uncomfortable with the mound.

I wonder if the differences are less pronounced for pitchers on a new team returning to their previous team's home park?

I hope you see what the relativity of home field advantages are to attendence, because my theory is that the roar of the crowd is a very significant factor. You might need to do this on a per stadium basis, because not all crowds cheer as much (Toronto was silent the last game I went to) and not all stadiums lend themselves to equal intensity (thinking of Fenway and Metrodome vs. Astrodome, etc.).

Note that home field advantage is stronger in sports with enclosed more completely surrounding arenas such in the NBA and NHL. Yet other than some grandfathered ice rinks, the playing fields in these sports are all identical.

Isn't there a rules advantage with home field? Walk Offs? 8 1/2 inning game strategy? Maybe I'm missing something, but it seems to me the rules are somewhat designed to give the home team a slight advantage, but the article doesn't address it.

Unquestionably players get more comfortable hitting and perhaps even fielding and pitching at home. Familiar environments, glares, backdrops, outfield angles, mound size(yes they vary from park to park), etc. all play a minute role on a home teams success. 4% sounds about right.

Is it possible that some of these statistics are swayed by the fact that visitors face the home team's better relief pitchers more often since they are frequently ahead? For example, I am sure the home team's closer (and theoretical best reliever) pitches much more frequently than his visiting counterpart.

It would be interesting to see if familiarity with the ballpark by a frequent visitor (e.g., division rival) lessens homefield advantage.

Another idea for a study on familiarity that Bill James proposed is the differences between a player's first year at a new park versus subsequent years.

I like the direction this is headed...

One thing to look at is whether team-specific HFA is correlated y2y (assuming no stadium change). I.e., is it more accurate to use a generic HFA for all teams, or to tailor it based on a team's historical HFA?

On a similar note, I hope you can extend the analysis at some point to look at individual player home/road splits and whether they have any predictive value (vs. using generic team home/road adjustments). I suspect past individual splits are mostly worthless for predicting future splits.

Matt is a top-three writer at BP already. Great stuff!

I'll tell Matt what I was told by a union guy during a summer job 11 years ago: "slow down, this is a union job...you'll make the rest of us look bad."

Great article Matt. I've always thought batter's eye is a huge factor, and it seems like you're pointing to that being one of the major reasons.

Someone upthread suggested looking at pitchers who are with a team for a while and then leave -- do they come back and perform better at their old haunt? I'd say the same study with hitters would be worthwhile. Someone who played in a stadium for a long time and then switches to another intra-division team would be good to study. He'd have lots of at-bats in the old stadium, but as a player on the other team. Does he still perform better at his old home? If it's about the batter's eye, then I'd have to think he'd have some advantage still (although the mound and outfield wall for the home team would be a bit of a counterbalance).

I think the data for triples is off by a factor of 10. "[T]he rate of triples per ball in play is 7.59 percent for home teams and only 6.44 percent for road teams." From context, I think you mean 0.759% for home teams and 0.644% for road teams. That's still significant, but no longer huge.

I would love to see this broken out by park--I would expect that oddball stadiums like Fenway Park or Baker Bowl would help their home teams more than Generic Ball Park.

1. Home team batters know the light, so they see better.

2. Home team pitchers know the mound, so the pitch better.

3. Home team fielders know the ground, so they catch better.

I hope the series will address whether the 54% home field advantage stays them same when both home and visitor are good teams (above .500).

About the "first-inning effect" - any thought as to whether the starting pticher having to sit through his team's at-bat after pre-game warmup might be having an impact?

The increase for innings 2-3 would suggest it's not big, but it was the first thing that came to my mind when I saw how much larger the first inning number is than ever those two.

I guess this is also realted to the idea of pitchers "cooling off" during a long batting inning, which is a topic for another study.

Referring to "large psychological effects" in baseball; "there seems to be little evidence of any strong, detectable effects, even if we know they exist" -- Err, uh, and just how do we know that they exist?? It seems to me that there are all sorts of prosaic explanations for the home team advantage -- teams tailored to the park, umpiring, knowledge of the park as it relates to positioning players, etc. Why do we have to believe there must be some psychological extract-of-moonbeam at play.

Maybe I'm being obtuse, and maybe this is addressed in later parts, but I'm not really sure any conclusions can be drawn from looking at stats that favor winners when trying to determine why winners won.

For example, striking out more batters and walking fewer batters increases your chance at winning games. Home teams won more games. Ergo, it would be expected that they struck out more batters/walked fewer. But was striking out more batters and walking fewer batters the cause (in part) of home teams winning more games or simply the effect of examining a data pool in which home teams won 54% of the games?

Or put another way, is it 1) home teams won 54% BECAUSE (in part) they struck out more/walked less; or 2) home teams struck out more/walked less BECAUSE they won 54% of the games examined? Which is the cause and which is the effect?

Reading this article, I cannot tell if it has been clearly established why the view point is 1) (which appears to be my reading of the article). Because unless I've missed it, I don't see how 2) has been eliminated as a possible conclusion which simply supports the notion that these are the stats that win games - and not the reasons why home teams win 54% of the time.

First off, great stuff again Matt. I especially liked the increased chance of run scoring by inning and the increased triple frequency.

If your hypotheses are true, is it possible to see if the same effects are noticeable in all-star games? This might serve also as a control element for travel and crowd/psychology issues.

Second, there's a bit of a myth that groundskeepers tailor the field to suit their team, either by sculpting the foul lines for/against bunts, trimming the grass more or less depending on the defensive characteristoics of a team, and dampening the basepaths to prevent stolen bases. I imagine that might be hard to measure, but if that myth is true, it might be a factor to consider.

Other factors to consider include home team video rooms might be better organized and equipped, and with pregame coaching might prep home teams better. Also the home team might have access to camera angles, for positioning, pitch location, and sign stealing that visitors don't have. In any event, the home team advantage might be more than psychological or rules based..