Saturday, April 23, 2011

Which Shots Are More Likely To Be On Target? Accurate Shot Creation in the Premier League

In an earlier post, I took a closer look at shot creation in the Premier League to see what number and proportion of shots are created from different match situations. The numbers, courtesy of the Opta/Guardian chalkboards and covering the first half of this year's EPL season, showed that most shots were generated from open play (11 on average per team/match). Teams also created chances from other situations, but at a much lower frequency (corners: 2.1, free kicks: .82, fast breaks: .52, penalty kicks: .14).

In terms of relative proportions, this means that roughly 75% of shots taken by Premier League teams in the first half of the season resulted from open play, 15% from corners, 5% from free kicks, and another 3.5% from fast breaks, with less than 1% resulting from penalties. Finally, there are some obvious differences across teams; to name just a couple of examples: Bolton were almost three times as likely as Liverpool to rely on free kicks to create shots, and Tottenham were twice as likely to generate shots from corners as Wigan.

So far so good; but as we well know, taking shots and shooting accurately are two different things. We can think of overall numbers of shots taken as a rough measure of offensive pressure, but real threats are only those shots that have a chance of hitting the back of the net. So the next step in understanding shot creation is whether some situations are more likely to yield accurate shots than others. Knowing the overall distributions of shot origins are helpful; for example, knowing that 75% of all shots come from open play would lead us to assume (absent any other information) that accurate shots are also more likely to be found in that category. But this could also be a fallacy: even though penalty kicks constitute fewer than 1% of all shots, it is easy to assume that most of them are on target.

This example highlights an important analytical consideration: while we may want to know where accurate shots come from - their absolute frequency - we may also want to know their relative threat - that is, the odds of any one shot being accurate. So below, I describe patterns in accurate shot creation in absolute and relative terms.

Let's start with the absolute frequency of shots on target. One thing to keep in mind: Opta's definition of shots on target includes only shots that really had a chance of going in (and we include actual goals in the count, of course). This means that we do not include blocked shots. So without further ado, here we go.

Using this way of measuring accurate and therefore highly threatening shots, it's clear right off the bat that accurate shots are a much rarer occurrence than total shots. On average, Premier League teams each managed about 4.4 accurate shots on goal per match. In total, about 3.4 accurate shots were created from open play, about .45 from corners, about .22 each from free kicks, and fast breaks, and .12 from a penalty. Another way to read these numbers is that teams produced about 1 accurate shot from a penalty every 8 matches, from fast breaks and free kicks every 4 1/2 matches, and from corners roughly once every other match.

While the overall numbers of highly accurate v. all shots considered in the other analysis are very different, their relative distributions look very similar. But to get a real sense of this, here are, first, the relative distributions of accurate shot creation (to one another), followed by the ratio of accurate shots to overall shots.

The vast majority - 77% - of all accurate shots are created from open play. This is slightly higher than the proportion of overall shots created from open play. Moreover, the data show that the relative importance of corners noted in overall shot creation is diminished when we look at accurate shots only, as about 10% of all accurate shots are created in the aftermath of a corner. Finally, the relative importance of free kicks and transition play in creating accurate shots on goal are about equal at 5%; as well, the relative importance of penalty kicks goes up threefold (from .93 in the overall shot distribution to 2.8% for accurate shots).

So far, so good. It's one thing to know that most accurate shots are created from open play; but it's another know which type of shot creation is associated with greater accuracy. So another way to slice these data is calculate with greater precision which type of shot creation is associated with greater shot accuracy.  To do that, we can calculate the ratio of highly accurate to all shots taken in the league; the number varies (theoretically) between 0 and 1, with 1 indicating that all shots created from a situation were on target and 0 indicating that none were. The next graph shows which type of shot creation led to shots that were most and least likely to be on target in the first half of the current EPL season.

Lo and behold, this is where we see some real differences across types of shots created, and these differences no longer reflect simple overall distributions. For starters, the overall average ratio of accurate to all shots is .3 (so slightly less than one in three shots were accurate). Shots created from open play are right around the average in accuracy, while free kicks are slightly below at .25. Unsurprisingly, shots created from penalty kicks were most likely to be on target at a rate of .88. In contrast, shots created from corner situations were least likely to be on target (at .2). Finally, and perhaps most interestingly, shots created from transition play (fast breaks) were the most likely to be accurate (ignoring penalty kicks for a moment), at a rate of .43.

To summarize these numbers more intuitively, here's what we can say if we round a little:
  • Overall, slightly fewer than 1 in 3 shots created are highly accurate
  • 9 of 10 shots created from penalties are accurate
  • More than 2 in 5 shots created from fast breaks are highly threatening to the other team.
  • 1 in 4 free shots from free kicks are accurate
  • 1 of 5 shots created from corners are accurate
So aside from penalties, fast breaks are most likely to generate high potential shots, while corner situations are least likely to do so. If you think about it and recall from the other post that the overall frequency of shots from fast breaks is about .5 per match, it implies a curious distinction: shots created with very low frequencies (penalties, fast breaks) have the very best odds of being accurate. In the next few days, I'll take a look at whether we see significant differences in creating accurate shots across teams and whether certain kinds of accurate shots are more likely to produce goals.