Recall from my earlier post that the 2009-10 Premiership season was unusually rich in goals. The season came within a whisker of having the most goals ever in the history of the league, and its 1,053 goals and 2.8 goals per match were among the highest ever.
So now that the first matches have been played in this year's EPL season (with the exception of Monday's Man U - Newcastle matchup), I thought it'd be useful to take a quick look at goal and shot ratios for 2009-10 to get us up to speed on how teams got there, and what this may hold for 2010-11.
Before looking at the data, recall that the ratios of SOT/Shots > goals/SOT > goals/shots. This means that shots are more likely to be on target than to yield goals, and that goals are more likely to be scored off accurate shots than just shots (none of this should be a surprise to anyone, but it helps to know this pattern when looking at the data).
So which teams were most and least trigger happy in the EPL last year? The chart below shows the average number of shots a team took per match, and it looks very similar to what the league table looked like most of the season. But if you look closely, there also are a few interesting exceptions.
On average, EPL teams took slightly more than 12 shots per match (12.22), and the range was as low as 8.34 for Hull and as high as Chelsea's 18.34. And it's no surprise to see that Chelsea, Man U, Tottenham, Arsenal, and Liverpool led the league in creating opportunities to take shots on goal. The team that's missing from this list of top clubs is Man City, which is very much in the middle of the pack when it comes to taking shots on goal.
But if all it took to stave off relegation was taking shots on goal, then Portsmouth would have easily been saved and Wigan wouldn't have been anywhere near the relegation battle, while Birmingham would have been in some trouble (which they weren't, of course).
But if all it took to stave off relegation was taking shots on goal, then Portsmouth would have easily been saved and Wigan wouldn't have been anywhere near the relegation battle, while Birmingham would have been in some trouble (which they weren't, of course).
These numbers are interesting, as far as they go. But as I've mentioned before, converting shots to goals is about more than randomly slinging the ball in the general direction of the opposite goalkeeper. Specifically, aside from taking shots, it requires shooting somewhat accurately to give the opposing keeper something to do and converting accurate shots into actual goals.
So here are the 2009-10 EPL ratios on goals, shots, and shots on target:
On average, there is a fairly narrow distribution in goal/shot ratios across teams (these measure how often teams score, given the number of shots they take). In 2009-10, the EPL followed Charles Reep's 9 shot maxim (that, on average, one goal is scored for every nine shots) almost to the decimal. Reep's prediction would yield a goal to shot ratio of .111, whereas the 2009-10 EPL season yielded a .116 ratio.
But averages don't win titles. You can see from the graph that the successful teams had a much higher yield around .15 (about one goal in seven shots), whereas the least efficient teams came in much lower. Portsmouth and Wigan's roughly .07 (yes, that's 1 goal in 14 shots) pale in comparison to Man City and Chelsea's .16 (that's 1 goal in 6.25 attempts).
So the question is this: is this difference in yields due to accuracy - do the Chelsea's of this world shoot more accurately? - or is it because of differences in conversion - do accurate shots taken by these team find the back of the net? As it turns out, it's clearly the latter.
Shooting accuracy (the green dots) around the league is at .55. But what's notable about the pattern of green dots is how closely together they are and that they do not follow the league table. Everton had the most accurate shooters (at > .6) and Bolton the least accurate ones (~ .53), but Chelsea's and West Ham's accuracy are virtually identical (and we know their season records weren't).
So while the distribution of SOT/shots ratios are fairly similar across the league, we see much bigger differences in conversion efficiency (look at the gap between the goals/SOT ratio and the simple goals/shots ratio - the blue and red dots): here, we see a big gap for the best teams, and a small gap for the not so good teams. To overstate things (only) a little bit, in the EPL, everyone shoots accurately* (the EPL is literally off the chart, as we have seen in the case of the Bundesliga, or the Big 4 leagues comparison, for example), but teams diverge when it comes to converting accurate shots into goals.
This past weekend, we already saw some of these patterns re-appear in the first match of the season, with a lively Aston Villa shooting more often than anyone else. Meanwhile, the big winners Chelsea and Blackpool shot more accurately and found the back of the net efficiently often than the other teams (with the exception of Sunderland who had only two shots on target and converted both, and Blackburn who had one shot on target and converted - thus bringing their goals/SOT ratio to 1).
Can't wait to see what the season brings. Happy shooting (and counting).
Can't wait to see what the season brings. Happy shooting (and counting).
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* This explains why the correlations between SOT and shots with goals and wins are similar for the EPL. Here are the correlations for 2009-10:
Goals Wins
Shots .39 .15
SOT .45 .19
So part of the reason the correlations between shots and SOT on on hand and outcomes on the other are so similar has to do with the accuracy of EPL shooters I reported on in an earlier post. Compared to the other big leagues, the EPL has the highest SOT/shots ratio.
