Examining Home-Field Advantage

1. Examining Home-Field Advantage Phil Birnbaum www.philbirnbaum.com

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

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

4. 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"

5. 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

6. "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

7. "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

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

9. Home team outscores visitors regardless

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

11. 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

12. 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

13. 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 …

14. 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!

15. 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.

16. 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

17. 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)

18. 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

19. 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

20. 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

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

22. 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

23. 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.

24. 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

25. 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.