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Score Effects in the English Premier League (and other leagues)

I'm guessing that most people coming to this blog are familiar with SB Nation's soccer coverage, but if you're not, I invite you to take a look at the main soccer page. SBN has six sites devoted to the English Premier League, five MLS sites and two general soccer sites, along with a huge amount of World Cup coverage.

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In general, home teams take 56% of all shots in games they play – 27% more shots than the visiting team. But this rate is not uniform – when a team is up, it will tend to play somewhat more defensively and allow more shots while taking fewer, and vice-versa. What this table shows is the percentage of shots taken by the home team by minute and by score:

Time Down 2 Down 1 Tied Up 1 Up 2
Total 58.1 60.1 56.0 52.3 54.9
1st Half 63.6 59.3 55.8 54.9 55.3
45-75 54.2 59.8 56.4 52.5 56.8
75-90 61.0 62.2 56.5 47.2 51.8
90+ Min 59.4 74.4 54.6 46.0 52.1

So overall, we see that the percentage of shots taken by the home team drops by 7% when they go up by a goal. Interestingly, their percentage actually goes up when they're up two goals. We don't see this effect in other (North American) professional sports, but scoring levels are so low in the EPL that a team does not necessarily need to play defensively when it's up by two goals. Also, the EPL has a bigger spread of team talent, and so when a team is up by two goals, it often means that it's so much better than its opponent that the threat of being scored on is quite small.

The shot rate also changes quite significantly over the course of the game. During the first half, the home team takes 55.8% of all shots with the score tied, and 54.9% when they're up one goal, which is less than a 2% drop. But in the second half, shot ratios are much different. From the 45th minute to the 75th minute, home team shooting is 7% lower when they're up a goal. And after the 75th minute, home team shooting is 16% lower when they're up one. We see similar effects when the home team is down one goal, and in injury time after the 90th minute, we see a particular desperation from the home team – they take three times as many shots as the visitors.

We also see score effects in shooting percentage – when the home team is up, they are more likely to score on a given shot:

Time Down 2 Down 1 Tied Up 1 Up 2
Total 9.6 9.3 9.5 11.8 12.0
1st Half 17.6 8.5 9.4 10.1 16.6
45-75 7.8 8.5 8.4 11.8 10.2
75-90 9.4 10.8 10.4 13.5 12.1
90+ Min 7.3 10.9 13.3 13.6 13.5

Shooting percentage is also higher in the last 15 minutes of the game, and significantly higher in the 90th minute. We see the same effect in the home team's save percentage:

Time Down 2 Down 1 Tied Up 1 Up 2
Total 86.6 88.4 91.9 91.5 90.8
1st Half 84.6 90.0 92.3 91.8 91.0
45-75 89.1 89.3 92.6 90.9 92.6
75-90 85.4 85.6 90.6 91.7 89.7
90+ Min 82.1 90.2 89.1 93.0 88.2

In particular, when the home team is down by one goal in the last 15 minutes of the game, opponents' shots are 50% more likely to go in. This reflects the strategies of both teams – the home team takes almost twice as many shots as the away team, but when the visitors do get an opportunity, it's often a very good one. This is because the home team is playing in a more offensive formation and has fewer players back to stop the opponent's rush.

The unsurprising bottom line: when a team is up one goal, it plays more defensively and allows more opportunities than when the game is tied. Because they have more players in defensive spots, the shots they give up tend to be better defended and are thus less likely to go in. And when the leading team gets an opportunity, it's a higher quality one because the other team doesn't have as many players back as it usually does.

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