Examining Home-Field Advantage

Phil Birnbaumwww.philbirnbaum.com

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

Inning Overall When one team has 4+ run lead

1 +18% more runs

2 +10 +19

3 +11 +10

4 +8 +13

5 +10 +9

6 +8 +8

7 +7 +8

8 +6 +5

Umpires and leverage Mitchel Lichtman finds some effect

of clutchness on called strike HFA, but less than "Scorecasting" found

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" Accounts for one-sixth of HFA

Dan Turkenkopf, "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 2000-2009 (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

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! 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."

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 topic

Mike Fast was especially helpful, pointing me to some of the studies mentioned here. Thanks, Mike.