Examining Home-Field Advantage Phil Birnbaum

Home Field Advantage (HFA) In baseball, home teams generally win 54% of games Why? Several possible explanations

Possible causes Fan enthusiasm Familiarity with park Molding team to park Travel "Home Cooking" Umpires Batting Last Others

Can eliminate some Fan enthusiasm Nope. Seems to be no correlation between HFA and attendance Familiarity Only a little Travel Nothing significant Batting Last Not just in close games Pitching last also has its advantages See "The Diamond Appraised" and "Scorecasting"

Recently: Umpiring "Scorecasting," by Tobias Moskowitz and Jon Wertheim, released early 2011 Claims refereeing/umpiring is the true cause of HFA Lays out some evidence for several sports

"Scorecasting" on umpiring Umpires call more strikes for home team pitchers than for visiting team pitchers The higher leverage (clutchier) the situation, the bigger the effect Umpires actually favor the visiting team in less-important situations

"Scorecasting" on umpiring Authors claim umpiring/refereeing accounts for almost all of HFA But … doesn't mesh with other evidence of the incidence of HFA

HFA appears in all situations For instance: When one team is ahead by 4+ runs early, that's low leverage. But HFA remains high

Home team outscores visitors regardless InningOverallWhen one team has 4+ run lead 1+18% more runs

Umpires and leverage Mitchel Lichtman finds some effect of clutchness on called strike HFA, but less than "Scorecasting" found Mitchel Lichtman Me too

Other estimates are lower John Walsh, "The Hardball Times Annual 2011" Home team favored by 0.8 pitches per game Accounts for one-third of HFA J-Doug, "Beyond the Box Score"Beyond the Box Score Accounts for one-sixth of HFA Dan Turkenkopf, "Beyond the Box Score"Beyond the Box Score Accounts for one-eighth of HFA

Not just umpires? There might be other things going on, not just umpires How can we find out? Look at things that don't involve umpires

Like what? Maybe … fielding. Once a ball is in play, umpires don't control whether it's a hit or an out If defenses turn more balls into outs at home, would that show that HFA is more than just umpiring? No, not really. Because …

Compensating for umpire bias … if umpires are more lenient towards home batters, they get more balls and fewer strikes More favorable counts Better hit balls Harder to field those balls So it could be umpiring after all!

Compensating for umpire bias Similarly in other sports In hockey, the referee's main influence is in calling penalties Home teams outscore visiting teams even at full strength But, it could be because visiting teams have to play less aggressively out of fear of referee sensitivity Same for soccer, basketball, etc.

How to tell? Need a measure of HFA that is not influenced much by umpires How about wild pitches and passed balls? Objective decision: ball eludes catcher, runners advance

WP/PB home field advantage For (God Bless Retrosheet): Home teams: 539 WP+PB per 100,000 pitches Road teams: 557 WP+PB per 100,000 pitches 3.3 percent difference Statistically significant (barely)

WP/PB home field advantage On 0-0 counts only Home: 511 WP+PB per 100,000 pitches Road: 544 WP+PB per 100,000 pitches Even larger effect

WP/PB home field advantage Overall difference: about 5 wins over 10 years Works out to.0002 wins per game That's 1/200 of HFA Runs resulting from WP+PB are 1/50 of total runs Not perfect, but reasonable

Basketball Free throw shooting percentage is not subject to referee bias HFA in foul shooting is about 0.2 percentage points in favor of the home team HFA in foul shooting Difference of 120 points a year 0.1 points per game Overall HFA is 3 points per game Again, seems reasonable

Speed skating There is a home-field advantage in speed skating! speed skating Of course, that can't be because of refereeing What could it be?

Another theory For some biological reason, humans just perform better at home than on the road Testosterone levels: "players may have tapped into a primal instinct to defend their own territory." Testosterone levels But, whatever: some intrinsic evolutionary reason is plausible, because HFA is pervasive and universal And none of the other hypotheses have tested out

Competing guesses on HFA "Scorecasting" Almost 100% umpires. Craig Wright, "The Diamond Appraised" (1989) 5% crowd … 5% last AB … 10% familiarity … 10% shaping team to park … 30% home cooking … 40% umpires. Me (among others) 10% umpires … 80% intrinsic/testosterone … 10% other.

Where to go from here? Need to come up with ingenious ways to decouple umpiring from other factors If you can think of any, let me know For now, I think there's good evidence that umpiring is only a small part of what's going on

Acknowledgements Thanks to readers at my blog for many comments, suggestions, and references on this topicmy blog Mike Fast was especially helpful, pointing me to some of the studies mentioned here. Thanks, Mike.