Wednesday, May 4, 2011

Why the Goal Value of Corners Is (Almost) Nil: Evidence From the EPL

A while back, I wrote about the goal value of corners. Turns out that more corners don't equal more goals. Across the big leagues, the correlation between corners and goals is essentially 0 (it's strongest in the EPL at .06, and weakest in Serie A and La Liga at less than .01). The graphical representation of this pattern tells the story, keeping in mind that the average number of goals a team scores per match is around 1.3 across leagues and seasons.

So, statistically speaking, the offensive "value" of corners seems to be slim to none. Similarly, match outcomes appear to be unaffected by corners. Assuming that earning corners is an indicator of offensive pressure, shouldn't teams that generate lots of corners also be teams that generate more goals? What gives?

One of the issues with the kind of analysis above, of course, is these data are based on match totals. As such, they are not designed to tell us the odds that any one corner actually yields a goal - they are simply match totals  "macro-level data", so to speak - that are aggregated over the course of an entire match.

To dig a little more into the goal value of corners, what we ideally would also want is more information about the micro-level of play; that is, what exactly happened after any one corner was taken and whether it yielded a goal. More specifically, we want to know what the odds are that any one corner kick ends up with a team putting the ball in the back of the net.

So to get a better handle on this issue, the good folks at StatDNA generously dug into their treasure trove of in-match data and helped me to calculate shots and goals scored from corners for a reasonably-sized sample of about 12-14 Premier League matches from this season. Shots and goals created from this particular match situation are defined here as occurring within three touches of a corner.

We can think of goals produced from corners as a simple chain of events: corners lead to shots, and shots lead to goals (of course, some corners are direct shots and thus some goals are directly scored from corners). So I wanted the know the following:
  • What proportion of corners actually produces shots on goal?
  • What proportion of shots created from corners produces goals
  • And overall, what is the ratio of corners to goals?
Of course, we would expect some slippage along the way. Not every corner will produce a shot on goal, and not every shot on goal will go in. As a consequence, the ratio of corners to goals is likely to be smaller than 1. "But how how much smaller?", inquiring minds will want to know. And where does most of the slippage occur?

Enough of the preliminaries. Here's what we see for teams in the Premier League.

First things first: here are the overall numbers of corners per team in this sample of matches. On average, teams took 5.35 corners per match. This is consistent with the five year league average of 5.52. While Wolves were particularly high at around 8 in this sample (for season as a whole this year, they are at 6.2 - also quite high), the rest of the teams ranged between a low of about 4 (Wigan) and 6 (Chelsea), with the better performing teams this year producing more corners.

While this is not what we're primarily interested in, it gives us a baseline against which we can compare the shot and goal production from corners. So here is the answer to question one; what are the proportions of corners actually producing shots on goal, according to the StatDNA data?

The data show that, on average, about 20.5% of all corners (that's 1 in 5) led to a shot by the team that took the corner within three touches of the ball. There also is considerable variation across clubs on this. At the low end, some of the very best clubs in the league managed relatively few shots - about 1 to 1.5 in 10 - relative to the number of corners they produced. In contrast, some of the worst teams in the league produced a relatively high number of shots in the aftermath of corners (Chelsea is the exception), at a rate of 1 in 4 or even 1 in 3 (West Ham and Stoke).

But clearly, there is some slippage; another way to think about this is that most corners did not yield shots (about 80% of them don't). Think about how different this is likely to be from free kicks. But on to question two. Of the corners that actually led to shots within three touches, what were the odds that the ball crossed the line? Take a look.

There is even greater slippage when it comes to turning shots into goals than turning corners into shots. In fact, it's almost twice as much. On average, about 11% or 1 in 9 shots created from corners produce goals. Incidentally, that's also right around the average ratio for goals to all shots taken in soccer matches (the Reep ratio). In this sample of matches, Fulham and Arsenal led the league in converting corner-created shots into goals at a rate of over .25, while at the low end, none of the shots created by Sunderland, Man City, Bolton, or Birmingham found the net.

So what do the patterns look like when we put all this together? The graph below of the ratio of corners to goals tells the combined story.

The yield from corners ranges from 0 to .07. On average, the data show that a corner is good for (drumroll ....) 0.022 goals. This means that the average EPL team scores 1 goal from a corner about every 10 games. And this helps to explain the lack of a correlation between the number of corners and goal scoring. The infrequency of the goals from corners combined with a lack of dispersion between teams in corners per game lead conspire to make corners mostly, well, useless when it comes to scoring goals.

A couple of caveats: since the sample of matches and teams is mixed, the data aren't really meant to be used to draw very strong conclusions about which teams are more skilled at corners than others. And unfortunately, the data analyzed here don't tell us whether corners were short or long. But the patterns reported here do raise some interesting questions about corners more generally and what this means for how teams should think about them. Assuming that most corners are played long and into the vicinity of the six yard box, perhaps the obvious implication is not to think of corners as goal scoring opportunities in the first place. Sure, there's something really impressive about a beautifully and accurately struck corner kick that leads to a header the goalie can't hope to get his hands on. But, as the data suggest, these moments are few and very far between. So instead, perhaps we should think of corners as opportunities to maintain possession in the final third of the pitch. So unless you've got a Peter Crouch-like striker lurking in front of the goal all by himself, you should think about taking short corners.