Home field advantage

In their book Scorecasting economist Tobias Moskowitz and Sports Illustrated columnist L. Jon Wertheim looked at many truisms in sport and attempted to prove their existence or otherwise.

One of the things they looked at was home-field advantage; did it exist and if so why? They took the five main sports in the US and worked out the percentage of games won by the home team in each. There was definitely a home team edge.

League Home team win %
MLB 53.9%
NHL 55.7%
NFL 57.3%
NBA 60.5%
MLS 69.1%


Next up they took some of the more usually held truisms as to why a home teams win; crowd support, being more familiar with your surroundings, lack of travel etc. They effectively ruled out any of these as reasons for the home-field edge and instead landed on one main reason; referees.

Referees are involuntarily affected by the home support. They give 50:50 decisions to the home team to, again involuntarily, appease the home support. This makes sense in the case of the MLS having the biggest home-field advantage as in such a low scoring sport the referee can have a larger impact on the final score. This notion was supported by a paper on home field advantage in the Bundesliga by Thomas Dohmen in which teams with a running track around the pitch (thus the ‘pressure’ from the home crowd is lessened) have a lesser home-field advantage.

So. Knowing all that how does it stack up for GAA football?

Rob Carroll(@gaelicstats) & Kieran Collins(@DKieranCollins) did a review of home-field advantage in GAA football (see here) and concluded that there was an advantage in the League with the home team winning 56.6% of the time from 2001 – 2010.

With the league about to start I decided to review home-field advantage and see if we could delve a little deeper.

The below table shows the winning percentage of home teams, by Division, from 1999-2012 inclusive. Though probably not scientifically correct I removed all games involving Kilkenny & Carlow as I felt these *might* skew the lower league returns.

Division Home team win %
Division 1 62.7%
Division 2 62.9%
Division 3 68.2%
Division 4 61.2%


The overall home-field winning margin of 64.1% is quite a bit above those recorded in the aforementioned review. The main explanation for this, which will be expanded upon below, is that the further you go back the greater the home-field advantage was.

A few initial thoughts

  • An overall home-field advantage of c64% makes intuitive sense. If the referee is the biggest determinant then you would expect a sport with amateur referees to be highly affected.
  • The referees have a big impact on GAA but again probably not to the extent that they do in soccer so having a home field advantage of c5% less than soccer again makes sense
  • I was surprised at the fact that Division3 was so far ahead of other divisions

The Division3 spike intrigued me so I started to look through the data and it became evident that there was an issue with how I had created the Divisions. The streamlined League, as we now know it, came into being in 2008. Prior to that we had 2 Divisions split into As & Bs. I had thus made Division 1A = Division1; Division 1B = Division2 etc. Looking at the games it didn’t feel as if the Divisions were as representative of strength as they are now. Thus I broke the home-field advantage into new league format (’08 – ’12) which has been in operation for 5 years and the old league format for 5 years prior to that (’03 – ’07)

League games ’08 – ’12

Division Home team win %
Division 1 54.3%
Division 2 53.6%
Division 3 62.9%
Division 4 52.4%

League games ’03 – ’07

Division Home team win %
Division 1 64.3%
Division 2 66.0%
Division 3 67.6%
Division 4 61.4%


Again Division3 stands out and I cannot for the life of me fathom it. Not only does it stand out but it is a relatively recent phenomenon

Above that however is the sea change in home-field advantage that the new league format has introduced. In the past 5 years home teams have won 8.5% less games compared to the previous 5 years. That is quite a significant drop.

Perhaps it is not the new league format that has led to this change but rather how the league is now viewed by teams and managers. There has been a change in emphasis in recent years with managers like Mickey Harte & Conor Counihan seeing the league as a competition worth winning in its own right. Or perhaps it is a mixture of the equalisation of team strengths in the new League format, alongside a change in emphasis, that has led to this drop. Either way home-field advantage has dropped considerably during the past 5 years.

Appendix

Home-field advantage for each county since the introduction of the new league structure (’08-’12)

County Home team win %
Cork 88%
Down 76%
Kerry 72%
Wexford 72%
Clare 67%
Dublin 67%
Sligo 67%
Tyrone 67%
Leitrim 65%
Monaghan 65%
Limerick 61%
Antrim 59%
Cavan 59%
Louth 59%
Offaly 59%
Kildare 56%
Donegal 56%
Longford 56%
Roscommon 56%
Tipperary 56%
Armagh 53%
Derry 53%
Meath 53%
Waterford 53%
Wicklow 50%
Westmeath 47%
Galway 44%
Fermanagh 44%
Laois 37%
Mayo 35%
Carlow 33%

Data
All the results were taken from gaainfo.com (a wonderful site). I did start to check the venues to make sure those teams listed as being at home were at home however venues start to become sketchy fro c2007 so gave that up. The results, and listing of home teams, did pass the test for the most recent years so that will have to suffice!

Pre ’03
For those of you eagle-eyed … the home field average for  ’08 – ’12 was 56%, for ’03 – ’07 was 64.6% yet the home field advantage from ’99 – ’13 was 64.1%. The reason for this is that the home-field advantage for ’99 – ’02 was 78.9%!!

League games ’99 – ’02

Division Home team win %
Division 1 76.1%
Division 2 74.3%
Division 3 84%
Division 4 83.1%

The drop from ’99 – ’02 to ’03 – ‘07 is for another day!

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