Saturday 21 July 2012

Taking Your Shots At Home and Away.

It's essential to return to old topics once more substantial quantity and quality of data becomes available. Tempting though it may be to sign off a subject in any field as fully understood, it is rarely the case and today's conclusions should always be questioned or sometimes endorsed with new material.

Home advantage is is a widely accepted facet of professional sport and can readily be demonstrated in football. Individual teams may occasionally produce superior away results in comparison to their home form even over an entire season, but this can happen through random chance in large groups of trials and away advantage rarely persists. Factors such as crowd pressure, referee bias  and familiarity with the surroundings  are plausible reasons why home field advantage may exist, but little hard data exists to back up these theories.

On average, over time home teams outscore their visitors by a ratio of around 1.4:1 in the Premier League and that superiority filters down into the shooting conversion rates and shooting accuracy figures. Last season home sides had 3.5 more shots per game than their opponents and one more shot on target. This shooting disparity is likely to contribute to the size of the advantage, but reasons behind the existence of the imbalance has to remain speculation. Heightened aggression from the home team as discussed here may lead to a more attacking outlook from the hosts and this coupled with a deliberate offensive tactical approach may force the visitors onto the defensive. But whatever the reason, we are increasingly in a position to quantify the effectiveness of each team's shooting attempts and that enables use to being to peel away at some of the onfield interactions.

Average Shooting Numbers for Home & Away Sides in the EPL 2011/12.

Shots per Game. Shots on Target per Game.
Home Sides. 16 5
Away Sides. 12.5 4

Shot numbers add some detail, but by looking at the area of the pitch from where each shot originated and comparing expected conversion figures with actual ones we can begin to understand something about the onfield conditions under which home and away teams take their shots. We saw here that Stoke, who tactically approach most games as if they are the visitors, balance poor shot numbers by creating better than average chances and we can make similar calculations for all away teams.

Over multiple seasons, home sides out shoot away sides by a similar proportion as that seen in 2011/12. The average position of a home shot is a couple of yards inside the box and 7 yards wide of the centre of the goal, away teams are about a yard further out and a yard wider. So away sides are shooting from marginally less advantageous positions. A more useful way to demonstrate both  the difficulty and scarcity of away shots compared to home ones is to plot frequency of shots sorted by distance from the goal.

A Shot Map For Various EPL Sides 2009/10-2011/12.

Every team that played in the EPL since 2009 are represented in the frequency plot and the profile of the home and away plots are very similar. The first peak comes about 4 yards in front of the penalty spot and the numerical supremacy of the home sides is well demonstrated. Once we move outside the penalty area the plots effectively merge. The peak at around 24 yards from goal can be easily explained by the number of direct free kicks, you can't have a free kick inside the 18 yard line and from much further out a shot quickly ceases to be a realistic option. Shot numbers outside the box are also very similar, indicating that away teams take a disproportionately larger proportion of longer ranged shots.

So initially we can confirm that away sides take less shots than home teams and we can add that these shots are on average attempted from further. This incremental improvement in our knowledge of venue specific shooting tendencies can be added to by further comparing the scoring and accuracy expectations of each shot with the actual outcome.

Expectancy Values verses Actual Values for Home and Away Shots in the EPL 2009-2012.

Goals per 100 Shots.
Shots on Target per 100 Shots.
Blocked Shots per 100 Shots.
Home Sides. 9.3 32 25
Actual Totals
Home Sides.
8.7 30 27
Away Sides. 8.4 31 26
Actual Totals
Away Sides.
9.1 34 24

Once shot position has been accounted for, home sides would expect to score almost nine and a half goals per 100 attempts and over a sample spanning nearly 30 different EPL teams and 3 of the most recent seasons and they fell short of that target by over half a goal. They were two shots per 100 shy of the expectation in terms of accuracy and two more shots per 100 were blocked compared to the model's prediction. So home sides underperfomed in all three areas. By contrast away teams overperformed the model in each of the three categories. Away sides were expected to score 8.4 goals per 100 judged on shot position (we saw they shoot from marginally poorer positions),but they managed just over nine.

So we can now pull the various threads together. Home sides outshoot their opponents and they create seemingly better chances. However, in reality over thousands of shots they under perform against a non venue specific model. Defensive strategy at the moment plays no part in the positional shot conversion model, so we can speculate that home shots are made more difficult be weight of defensive numbers in and around the shooting area. The reverse may be true of away teams, they perform better than expected with their more limited number of shots. Again we may speculate that home team commit more to attack, allowing more freedom to visiting attackers when they do get within range.

If visitors chose or are forced into playing a more defensive game, they appear to reap a slight payback through a better than expected overall conversion rate for their goal attempts. They score slightly more, see more shots on target and less blocked than predicted by a venue neutral model and with even more detailed data we may soon be able to tell if this approach is close to the optimum strategy to deal with a pumped up home foe.

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