Monday 24 March 2014

Daniel Sturridge in the Penalty Box with a Football

In this post I am going to take an even closer look at Daniel Sturridge's shot performance, this time by location. I do not think goalscoring is random. In a post I did a month or so ago looking at variation of shooting success from the 50 most frequent shooters over the last few season the distribution looked decidedly normal. However, the variance is so large that it could appear random. After my post last week looking at Sturridge's shot data it struck my that this same large variation will be present in his data and it'd be worth breaking it down further to eliminate as much 'noise' as possible.

Expected goal models based on shot location are prone to error. For example, there are a lot of shots from outside the box in football, roughly half of all shots. For the data I have on Sturridge's 230 shots 80 of these have been outside the box (34%). The expected goal value for such a shot is 3-4% depending on the type of pass for the shot. A couple of goals outside the box can really inflate a player's conversion numbers, especially with small samples sizes. I have chosen deliberately to use samples of 20 just shots in this analysis with one eye on application. Things change so fast in football, and I believe stats really need to keep up with this to be meaningful.

I also imagine there will be relatively high error/variation with shots very close to goal (six yard box). This is me talking, not the stats (although I'll be sure to examine the data), but it strikes me that scoring from a few yards is more about positioning, being in the right place at the right time, and very basic technique, rather than an inherent skill to accurately guide a football past a goalkeeper. 

My logic is that shots in the 6 yard box will be amongst the most polarized, either a player is presented with a very simple "tap-in" type chance, where the goalkeeper is out of position, or a chance falls to a player in close proximity to the 'keeper or defender and the actual scoring angle is really difficult. Both shots will be given the same expected goal value. Without data on goalkeeper/defender location  I believe any expected goals model will really struggle to capture what's going on here and thus will be error-prone. It should be possible to benchmark the expected variance by shot location and I'll be sure to follow this up for all the different shot zones.

For this reason I have decided to focus in this post on shots in the main penalty area and exclude shots in the 6 yard box and shots from outside the box. Not that these aren't important, of course they are, I just think there are quite different factors at play with them both and they should be treated separately. I'm also ruling out headed attempts. Again this is just a personal hunch which I will evaluate properly in due course, but I would assume headers have a different statistical profile to regular shots when it comes to accuracy, placement and goal conversion. This shouldn't impact Sturridge's shot data too much though, so let's get back to him and his shot history broken down by two key locations in the penalty area.

Daniel Sturridge in Penalty Box (Centre)

All these charts show a rolling average of 20 shots. Sturridge has had 46 shots of his 230 from the central area of the penalty box. Only 14 of these were whilst at Chelsea were he had 102 shots in total (13%). Since joining Liverpool he's had 128 shots, 32 in Box (Centre) (25%). 

There's a lot less variation in this Goal Conversion chart than the one in my last post looking at all shots. There Sturrige's Goal Conversion reached 300% of the average. He does a great job in the central box but he's not been superhuman. 

Comparatively, there's is little variation in his shot accuracy. These charts show shot accuracy adjusted by expected accuracy, where 100% would be performing to league average by location, etc. There's a general 'eyeball trend' between shot accuracy and goal scoring, although the R2 is only 0.4. He is good though, never falling below average and we can see his 'shots in corner' accuracy has perhaps driven his increased goal scoring of late. Precisely how good though I cannot say and I will need to benchmark performance of all players shooting in the different locations. A quick t-test against accuracy of all shots in this location gives 97% confidence that Sturridge is better than average.

Daniel Sturridge in Penalty Box (Wide)

Whoah, this tells more of a story. Sturridge joined Liverpool from mark 13. Again this is a rolling average of 20 shots. He's clearly been on an upward trajectory since putting on a red a red shirt (once he got going) and it would seem a supreme level of shot accuracy has been the driving factor. 

What could explain this? My first thoughts are that it is team related. Liverpool are obviously tearing it up at the moment, so this will be the subject of my next where I'll track Liverpool's performance in conversion and accuracy in these two shot locations and compare that with Sturridge, as well as try to normalise any team effect seen.

To wrap-up, there's a more to go on looking at Sturridge's goal and shot data broken down  by position than there was in my first post looking at all shots bundled together. As usual though, this has given me more questions than answers. But that's a good thing,  Analysis is the process of breaking down a complex topic into smaller parts to gain a better understanding of it, and breaking down a player's shot data like this has given me a better understanding, or at least the next set of question to answer.

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