Friday, May 20, 2011

The Uselessness of Free Kicks in the Premier League

If you've been reading this blog these last few weeks, you know that I've been spending way too much time digging through data on shot creation in the Premier League with the help of the Opta/Guardian chalkboards. But I can't quite help myself, so here's yet another installment; this time it's on the (relative) uselessness of free kicks.

Don't get me wrong; I love a beautifully curled shot from 20 yards out into the upper right corner of the goal as much as the next guy. What I don't like is how rare and inefficient free kicks are, relative to other shot situations in a match. How rare and inefficient? To answer that question, we need information about the frequency of shots from free kicks and the efficiency of goal creation.

First, consider the graph below. It shows the overall frequencies of shots generated from free kick situations for the league as a whole, with the y-axis displaying the % of matches with various numbers of shots from free kicks situations. For the league as a whole, in 80% of all matches, teams have 0 or 1 shot on goal generated from a free kick. Compared to the average number of 14 overall shots per team and match in the first half of the season, shots from free kicks are very rare at a rate of just .82 per team and match.

Naturally, there are some interesting differences across teams. Take a look.

Aston Villa and Wolves took the cake with 76% and 70% of matches, respectively, in which they generated zero shots from free kicks. In contrast, in 83% of their matches, Wigan were able to produce at least one shot on goal, and Bolton, Chelsea, and Tottenham were able to shoot at least once in 77% of their matches.

So how accurate were these shots, and how likely were they to find the back of the net? As a benchmark, recall from the earlier analyses that PL teams on average produced 1.35 goals per match during the period investigated. I've already reported that free kicks contributed only .038 to the 1.35 average total; that's only 3% of the total!

So how do we get to these 3%? As we already know from looking at shots from fast breaks, rare doesn't have to mean inefficient. So to get a handle on the relative efficiency of shots from free kicks, it helps to look at shot accuracy and conversion.

Let's start with the frequency of accurate (as opposed to all) shots created from free kicks. For the league as a whole, the number of accurate shots on goal from free kick situations was .22 per team and match. That's significantly lower than the overall number of shots from free kicks (.82). But this average hides significant variation across teams. The league had one team that outdid all the others on this score: Chelsea who were far and away the best team when it came to creating accurate shots from free kicks, at a rate of around .7 per match. The next best team was Bolton at a rate of around .4 per match.

At the low end, a good number of teams were right around a lowly .1 accurate shots created per match from free kicks, and this included great and not so great teams. So it took some of these teams 10 matches to create one highly accurate shot on goal from free kicks, a hard-to-believe rate of production. This also means that lots of teams were below the league average (and that Chelsea's numbers brought up the league average, of course).

How efficient were teams in generating accurate shots? That is, how did the number of accurate shots relate to the overall shots teams created from free kicks? So here are the ratios of accurate shots relative to all shots created from different situations.

Recall that the overall average ratio of accurate to all shots is .3; turns out that shots created from free kicks were slightly below at .26. So teams were able to make 1 in 4 shots from free kicks accurate. Not bad, but not amazing, either. These ratios tell us how efficient or profligate teams were overall with the chances they did create from different match situations.

Keeping this in mind, how did teams compare at the season's halfway mark? As before, teams' levels of accuracy from free kicks varied quite a bit. Arsenal were among the league's worst performers. While they generated a middling amount of shots from free kick situations overall, they were unable to generate much threat from them and were bottom of the league in accuracy. In start contrast, Blackburn and Birmingham had much higher accuracy rates and topped the league. Chelsea, Liverpool, and Wolves also did quite well on this front.

But it's not enough to be accurate; teams also have to convert. So here are conversion rates (goals from free kicks to all shots on target from free kicks) for free kick situations (keeping mind that the league's average conversion ratio for free kicks was .18).

Here, we see that Aston Villa topped the league in conversion; when they did have accurate shots on goal from a free kick, they scored. The only trouble is that in this dataset, this happened exactly once - not quite enough to make much of a dent. At the other end, what is truly striking is that half the league failed to score from accurate shots generated from free kick situations. 

So what does it all translate to when we combine accuracy and conversion? This is where we get to the overall yield of goals from shots. And it doesn't look very pretty. About half the league were able to turn free kick situations into goals; about half the league weren't. Blackburn topped the chart at .25, and all others with positive yields were well below between (roughly) .05 and .15.

So no matter how many free kick situations teams were able to produce, the net result was skimpy overall, as the next graph shows. It displays the average number of goals per match produced by teams from free kicks. Turns out, free kicks were most useful to Wigan, Chelsea, and Blackburn in terms of actually producing goals. In contrast, some of the very best teams in the league hadn't scored from free kick situations at all by the end of December.

I realize that these patterns raise as many questions as they answer, and I can't do all the subtleties justice here. I'm also sure supporters of different clubs will see patterns I haven't picked out (or the odd mistake), and I welcome you to note them in the comments section. Lots of caveats apply, too. For example, the data say nothing about tactical reasons for the patterns we see; and absent systematic information about this, we have to guess. It also is not clear to me whether the patterns described here have held in the second half of the season as managers have adjusted to teams' successes and failures. And to be honest, to understand something as infrequent as free kick situations may very well require a larger sample than I have worked with here. From a statistical standpoint, rare events and small sample sizes are natural enemies.

Finally, free kicks are not quite like other match situations - say, shots produced from open play or fast breaks - because they are dead ball situations. As such, they perhaps aren't directly comparable to other types of shot creation. Fair enough. But their relative infrequency, coupled with dodgy odds of turning them into goals hopefully have done one of two things: either they have convinced you that they're not particularly effective devices for scoring in the Premier League, or at least they will make you think twice next time you get excited when your team is awarded a free kick not too far from the penalty box.

Which reminds me: why do we get excited about these match situations? I suspect it's because scoring from a nice free kick is so spectacular and such a beautiful thing to behold. And it looks great on highlight shows. But it's also a really hard thing to do, it turns out.