Examining Home-Field
Advantage
Phil Birnbaum
www.philbirnbaum.com
Home Field Advantage (HFA)
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In baseball, home teams generally win
54% of games
Why?
Several possible explanations
Possible causes
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Fan enthusiasm
Familiarity with park
Molding team to park
Travel
"Home Cooking"
Umpires
Batting Last
Others
Can eliminate some
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Fan enthusiasm
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Familiarity
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Nothing significant
Batting Last
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Only a little
Travel
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Nope. Seems to be no correlation between HFA and
attendance
Not just in close games
Pitching last also has its advantages
See "The Diamond Appraised" and "Scorecasting"
Recently: Umpiring
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"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
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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
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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
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For instance:
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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
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Mitchel Lichtman finds some effect of
clutchness on called strike HFA, but less
than "Scorecasting" found
Me too
Other estimates are lower
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John Walsh, "The Hardball Times Annual
2011"
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J-Doug, "Beyond the Box Score"
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Home team favored by 0.8 pitches per game
Accounts for one-third of HFA
Accounts for one-sixth of HFA
Dan Turkenkopf, "Beyond the Box Score"
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Accounts for one-eighth of HFA
Not just umpires?
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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?
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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
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… 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
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Similarly in other sports
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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?
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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
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For 2000-2009 (God Bless Retrosheet):
Home teams:
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Road teams:
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539 WP+PB per 100,000 pitches
557 WP+PB per 100,000 pitches
3.3 percent difference
Statistically significant (barely)
WP/PB home field advantage
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On 0-0 counts only
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Home: 511 WP+PB per 100,000 pitches
Road: 544 WP+PB per 100,000 pitches
Even larger effect
WP/PB home field advantage
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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
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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
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There is a home-field advantage in
speed skating!
Of course, that can't be because of
refereeing
What could it be?
Another theory
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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
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HFA is pervasive and universal
And none of the other hypotheses have tested out
Competing guesses on HFA
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"Scorecasting"
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Craig Wright, "The Diamond Appraised"
(1989)
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Almost 100% umpires.
5% crowd … 5% last AB … 10% familiarity … 10%
shaping team to park … 30% home cooking …
40% umpires.
Me (among others)
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10% umpires … 80% intrinsic/testosterone …
10% other.
Where to go from here?
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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
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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.