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| (c) 2010 Premier League |
Efficiency is an interesting thing. Analyzing football in terms of efficiency gets us away from just counting up good or bad stuff that happens on the pitch. Instead, it allows us to compare how well or badly teams or players made use of the footballing resources they had available. It won't surprise anyone to hear that Chelsea produced more shots than Birmingham that day. But it will surprise people that Chelsea were incredibly profligate, while Birmingham managed to make the most of what they had.
Speaking more technically, efficiency means connecting the inputs and outputs teams produce on the pitch. Take shots (inputs) and goals (outputs), as in the Birmingham-Chelsea example above. Birmingham's efficiency rate was 1, while Chelsea's was .04. The Birmingham Blues were literally 25 times as efficient as the Blues from southwest London.
Though it's a great example, this particular match also happens to be an outlier - an unusual occurrence. Goals to shots ratios are typically much more similar across teams in the top leagues. Commonly, some of the best teams outperform the worst ones by a 2:1 margin on efficiency (rather than 25:1). And some of the very, very best teams like Bayern Munich or Barcelona managed to produce goal/shot efficiency rates almost 3 times as good as some of the very worst like Bari or Sampdoria.
Part of what made the Birmingham-Chelsea match so interesting is that the home team was clearly much more efficient than the away team. Is this a pattern we should expect to see elsewhere? More specifically, are teams more efficient when playing in front of their own supporters than playing away from home? We know that home teams typically perform better than the away side in an absolute sense. They tend to shoot, score, and win more, for example (though the jury is still out on why exactly that is). But that's not the question here. Instead, the question is whether home teams are also more efficient - that is, whether they perform better in terms of connecting inputs to outputs.
Honestly, it's not clear to me that they should be. It's one thing to imagine that home teams, pushed along by a friendly crowd, referee, or familiarity with the home field environment, will produce more chances in an absolute sense. But why, given a chance, would a striker be more likely to turn that same chance into a goal, or a goalkeeper be more likely to prevent one at home than on the road? Yet that is what we seem to see, as the next graph with data from the 2010/11 season for the Premier League reveals.
So is this is something we see consistently across most teams? The quick answer is, well, not really, as the next chart shows (except if you're a Newcastle).
It appears that there are about as many teams that are more efficient at home, as there are teams that are more or about as efficient playing on the road. The former include Wolves, West Ham, Sunderland, Stoke, Newcastle, the Manchester teams, Liverpool, Fulham, and Bolton. The latter include West Brom, Tottenham, Everton, Blackburn, and Arsenal. And then there are teams that were about as efficient at home as on the road (Wigan, Chelsea, Blackpool, and Villa).
So, yes, there's a slight preponderance of teams being more efficient at home, but the difference seems slight, applies to about half of all teams, and there's plenty of exceptions.
Unfortunately for Chelsea, they ran into one of those exceptions last weekend yet again. The Blues scoring on 3 of 12 attempts means they did much better than in that fateful November match at St.Andrews. The only downside was that Arsenal's scoring with 5 of their 13 shots revealed them to be incredibly efficient on the day; as the data for last season show, the Gunners seem to have a knack for efficiency when playing away from the Emirates.
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| (c) 2011 Premier League |
