Friday, 21 February 2014

Variation in Player ExpG Conversion across the Premier League

In my earlier post today I dropped in a table containing "finishing" stats for 100+ Premier League players across 2.5+ seasons. I put finishing in quotations marks there as this stat is not really what most people will naturally understand as finishing, the data includes long shots, headers, and no doubt an unintentional ricochet off the odd backside. What it actually is a the ratio of a players goals scored to the average expected goals scored (ExpG) based shot location and type and pass type.

The bar charts in the table represent the "finishing" or ExpG Conversion of successive and unique buckets of 40 shots.  A good thing about these 40 shot buckets is that there are very few 0% conversions, i.e. when a player scores 0 goals from 40 shots, at least among the most frequent 55 shooters I've looked at here. There were just 4 instances out of the 158 player-bucket samples where the ExpG conversion was 0%

On doing this table a month or so ago I immediately saw an opportunity to test for the repeatability of ExpG conversion and @11tegen11's post recently 'How to Scout a Striker',  where he did a similar thing, spurred me on to get this done.

For every player with at least 3 x '40 shot buckets' (i.e. 120+ shots since 2011/12) I have plotted below a scatter plot of their nth bucket with their nth+1. From the main table this data is for the first 55 players when sorted by total shots (as it is by default), from Luis Suarez to Kevin Mirallas.



It's a mess, right. No correlation at all between a player's conversion rates from one set of 40 shots to his next 40 shots, and indeed a very similar result to that told by @11tegen11 in his post.

When I looked at the data in the main table though I couldn't help notice a few things. It would be handy if you opened the table in a new window and used the search box to take a look. Players who have notched enough shots to see some sort of trend, like RVP and Suarez, seem to have a generally stable conversion rate with an occasional high or low. Some players have higher highs or lower lows too. Additionally, some player can be seen to be improving pretty dramatically (see Sturrdge, Lallana, Rodriguez) whilst some appear to peter out (Defoe, Dempsey, Lampard). Other players can have enjoy a super purple patch (Cabaye, Adam Johnson) or bad patches (Bale, Ba, Dzeko).

Basically, look through the table at individual players I got the feeling that there's far too much not being accounted for with single players to make the above measure of repeatability a successful one. There's improvement with age (Sturridge/Lallana?), adaptation to the Premier League (Lallana again?). There's getting old, getting injured, or getting both. Changing clubs, changing managers, team mates injured or leaving. Lots. 

This made me wonder, unable to control for all these factors, what kind of conversion rate is normal or 'acceptable' in the Premier League. I took all the buckets and plotted the distribution,


As this appears normally distributed I went ahead and created a randomly generated set of normally distributed data with the same mean (1.03), standard deviation (0.499) and n (158) and plotted the "repeatability" of this data set from n to n+1 to mimic what I did for the shot buckets. Note this is a frequency histogram.






The same messy pattern and no correlation between one result and the next. But of course!. The individual points are all independent events. One result does not influence the next - could the same be said of shots?

What does this mean? Well, I don't know to be honest, perhaps a more learned statistician/football dude can offer some ideas, but a few thoughts spring to mind.

Without being able to control for things like age and injury it does not seem appropriate to asses or judge players ability to repeat their conversion rates, or expect to repeat a conversion rate when such a large variance seen. 

Instead perhaps players can be evaluated based on their own distribution within the league distribution, are they consistently better than average, what is their variance?  Perhaps method could be borrowed from statistical process control (SPC). For example, control charts with limits and zones in which to track a player's ability to score from his shots. 

Quick example knocked up below for Rooney and RVP's last 280 shots. Although both striker's conversion can vary a lot from bucket to bucket they both are basically just varying within the middle 'average zone'.  Both are 'in control', as they say. Could you really say one is better than another? I don't think so. 

Also, as RVP is considered one of the best strikers around and has scored the most goals, this backs up a general assertion lately that getting lots of shots in on goal is perhaps the real skill, rather than putting them away.



Once thing I think that can be said looking at ExpG conversion this way is Daniel Sturridge's current form is off the charts! Since this was done in GW22 he's scored another 5 goals. It'd be interesting to see what his blip below 50% was due to - was it just variation or was there a special cause assignable? Not in the mood at Chelsea perhaps? Anyway, time for lunch.





Thursday, 20 February 2014

Premier League Expected Goal Conversion Stats

Take a look at this post for an explanation of what this table means. In brief, it's data from 2011/12 to present (GW22 2013/14) on player actual to expected goals scoring rates. The table is sorted by Total Shots by default, click on column headers to sort further. The bar charts are consecutive but distinct 40 shot buckets and you can hover over them to see their values.
  Player Shots Goals xG Finishing Goalscoring                    
1 Luis Suárez 407 50 39.6 1.26 10.4
 
 
 
 
 
 
 
 
 
 
2 Robin van Persie 346 55 41.2 1.34 13.8 x x
 
 
 
 
 
 
 
 
3 Gareth Bale 304 28 20.6 1.36 7.4 x x x
 
 
 
 
 
 
 
4 Wayne Rooney 294 35 26.5 1.32 8.5 x x x
 
 
 
 
 
 
 
5 Sergio Agüero 273 42 32.9 1.28 9.1 x x x x
 
 
 
 
 
 
6 Demba Ba 259 28 27.1 1.03 0.9 x x x x
 
 
 
 
 
 
7 Edin Dzeko 230 34 28.3 1.20 5.7 x x x x x
 
 
 
 
 
8 Clint Dempsey 218 22 20.3 1.09 1.7 x x x x x
 
 
 
 
 
9 Adel Taarabt 214 6 9.6 0.62 -3.6 x x x x x
 
 
 
 
 
10 Daniel Sturridge 210 32 22.4 1.43 9.6 x x x x x
 
 
 
 
 
11 Theo Walcott 196 27 21.2 1.27 5.8 x x x x x x
 
 
 
 
12 Jermain Defoe 196 23 21.9 1.05 1.1 x x x x x x
 
 
 
 
13 Frank Lampard 182 20 13.1 1.53 6.9 x x x x x x
 
 
 
 
14 Olivier Giroud 177 19 21.8 0.87 -2.8 x x x x x x
 
 
 
 
15 Yohan Cabaye 176 12 8.1 1.48 3.9 x x x x x x
 
 
 
 
16 Andy Carroll 170 9 17.0 0.53 -8.0 x x x x x x
 
 
 
 
17 Yaya Touré 167 18 13.0 1.38 5.0 x x x x x x
 
 
 
 
18 Fernando Torres 167 17 18.5 0.92 -1.5 x x x x x x
 
 
 
 
19 David Silva 164 14 16.8 0.83 -2.8 x x x x x x
 
 
 
 
20 Romelu Lukaku 163 24 19.5 1.23 4.5 x x x x x x
 
 
 
 
21 Leon Osman 162 12 12.8 0.94 -0.8 x x x x x x
 
 
 
 
22 Stéphane Sessegnon 162 16 14.7 1.09 1.3 x x x x x x
 
 
 
 
23 Marouane Fellaini 161 14 16.6 0.84 -2.6 x x x x x x
 
 
 
 
24 Emmanuel Adebayor 160 24 24.3 0.99 -0.3 x x x x x x
 
 
 
 
25 Santiago Cazorla 158 15 10.4 1.45 4.6 x x x x x x x
 
 
 
26 Steven Fletcher 155 26 18.4 1.41 7.6 x x x x x x x
 
 
 
27 Steven Gerrard 154 9 9.0 1.00 0.0 x x x x x x x
 
 
 
28 Rickie Lambert 153 14 12.6 1.11 1.4 x x x x x x x
 
 
 
29 Gylfi Sigurdsson 152 12 11.0 1.09 1.0 x x x x x x x
 
 
 
30 Danny Welbeck 152 18 17.9 1.01 0.1 x x x x x x x
 
 
 
31 Jonathan Walters 150 10 14.4 0.69 -4.4 x x x x x x x
 
 
 
32 Papiss Demba Cissé 149 21 18.0 1.17 3.0 x x x x x x x
 
 
 
33 Aaron Ramsey 148 11 14.1 0.78 -3.1 x x x x x x x
 
 
 
34 Juan Mata 146 16 12.6 1.27 3.4 x x x x x x x
 
 
 
35 Christian Benteke 145 21 12.7 1.65 8.3 x x x x x x x
 
 
 
36 Michu 145 20 15.4 1.30 4.6 x x x x x x x
 
 
 
37 Stewart Downing 145 3 9.3 0.32 -6.3 x x x x x x x
 
 
 
38 Jay Rodriguez 144 15 13.3 1.12 1.7 x x x x x x x
 
 
 
39 Peter Odemwingie 141 15 12.1 1.24 2.9 x x x x x x x
 
 
 
40 Victor Moses 141 8 13.1 0.61 -5.1 x x x x x x x
 
 
 
41 Nikica Jelavic 141 15 15.3 0.98 -0.3 x x x x x x x
 
 
 
42 James Morrison 141 9 8.2 1.09 0.8 x x x x x x x
 
 
 
43 Gabriel Agbonlahor 139 17 13.1 1.30 3.9 x x x x x x x
 
 
 
44 Charlie Adam 138 8 7.0 1.14 1.0 x x x x x x x
 
 
 
45 Carlos Tévez 136 13 12.3 1.06 0.7 x x x x x x x
 
 
 
46 Peter Crouch 135 21 18.1 1.16 2.9 x x x x x x x
 
 
 
47 Franco Di Santo 134 12 10.3 1.17 1.7 x x x x x x x
 
 
 
48 Mohamed Diamé 134 8 9.1 0.88 -1.1 x x x x x x x
 
 
 
49 Danny Graham 133 15 16.8 0.89 -1.8 x x x x x x x
 
 
 
50 Dimitar Berbatov 130 18 14.2 1.27 3.8 x x x x x x x
 
 
 
51 Grant Holt 130 20 15.5 1.29 4.5 x x x x x x x
 
 
 
52 Andreas Weimann 125 11 13.3 0.83 -2.3 x x x x x x x
 
 
 
53 Craig Gardner 124 4 5.1 0.78 -1.1 x x x x x x x
 
 
 
54 Kevin Mirallas 121 9 8.5 1.06 0.5 x x x x x x x
 
 
 
55 Ramires 121 11 9.4 1.17 1.6 x x x x x x x
 
 
 
56 Hugo Rodallega 119 4 9.9 0.41 -5.9 x x x x x x x x
 
 
57 Chris Brunt 118 5 6.1 0.82 -1.1 x x x x x x x x
 
 
58 Mario Balotelli 116 11 11.0 1.00 0.0 x x x x x x x x
 
 
59 Nathan Dyer 116 12 8.4 1.43 3.6 x x x x x x x x
 
 
60 Samir Nasri 113 10 10.9 0.92 -0.9 x x x x x x x x
 
 
61 Hatem Ben Arfa 113 8 6.0 1.32 2.0 x x x x x x x x
 
 
62 Jordi Gómez 111 6 8.2 0.73 -2.2 x x x x x x x x
 
 
63 Jason Puncheon 111 8 8.7 0.92 -0.7 x x x x x x x x
 
 
64 Shane Long 110 18 13.5 1.33 4.5 x x x x x x x x
 
 
65 Kevin Nolan 110 12 15.8 0.76 -3.8 x x x x x x x x
 
 
66 Adam Johnson 109 14 6.0 2.33 8.0 x x x x x x x x
 
 
67 Bradley Johnson 108 5 5.5 0.91 -0.5 x x x x x x x x
 
 
68 Jordan Henderson 108 7 8.6 0.81 -1.6 x x x x x x x x
 
 
69 Robert Snodgrass 107 3 5.7 0.53 -2.7 x x x x x x x x
 
 
70 Scott Sinclair 107 5 9.5 0.52 -4.5 x x x x x x x x
 
 
71 Eden Hazard 104 16 8.0 1.99 8.0 x x x x x x x x
 
 
72 Rafael van der Vaart 104 10 9.3 1.08 0.7 x x x x x x x x
 
 
73 Javier Hernández 101 20 14.3 1.40 5.7 x x x x x x x x
 
 
74 Anthony Pilkington 100 12 9.4 1.28 2.6 x x x x x x x x
 
 
75 Steven Pienaar 99 11 7.7 1.42 3.3 x x x x x x x x
 
 
76 Philippe Coutinho 99 5 7.9 0.64 -2.9 x x x x x x x x
 
 
77 Loïc Remy 99 16 9.4 1.70 6.6 x x x x x x x x
 
 
78 Nani 98 9 6.4 1.40 2.6 x x x x x x x x
 
 
79 Oscar 98 9 7.8 1.15 1.2 x x x x x x x x
 
 
80 Jonjo Shelvey 97 5 6.5 0.77 -1.5 x x x x x x x x
 
 
81 Arouna Koné 95 11 11.8 0.93 -0.8 x x x x x x x x
 
 
82 Chris Eagles 95 4 5.7 0.70 -1.7 x x x x x x x x
 
 
83 Shaun Wright-Phillips 94 1 6.0 0.17 -5.0 x x x x x x x x
 
 
84 Steve Sidwell 93 9 7.8 1.15 1.2 x x x x x x x x
 
 
85 Victor Anichebe 91 11 9.2 1.20 1.8 x x x x x x x x
 
 
86 Glen Johnson 89 2 6.2 0.32 -4.2 x x x x x x x x
 
 
87 Ashley Young 88 7 7.2 0.97 -0.2 x x x x x x x x
 
 
88 Bryan Ruiz 88 7 8.0 0.87 -1.0 x x x x x x x x
 
 
89 Adam Lallana 87 8 8.3 0.97 -0.3 x x x x x x x x
 
 
90 Robert Huth 86 4 9.4 0.43 -5.4 x x x x x x x x
 
 
91 Luka Modric 83 4 5.1 0.79 -1.1 x x x x x x x x
 
 
92 James McClean 76 6 6.3 0.95 -0.3 x x x x x x x x x
 
93 Wes Hoolahan 76 7 6.4 1.09 0.6 x x x x x x x x x
 
94 Matthew Jarvis 76 10 7.3 1.38 2.7 x x x x x x x x x
 
95 Bobby Zamora 76 10 7.5 1.34 2.5 x x x x x x x x x
 
96 Daniel Agger 75 5 6.8 0.74 -1.8 x x x x x x x x x
 
97 Shola Ameobi 74 3 5.9 0.51 -2.9 x x x x x x x x x
 
98 Louis Saha 74 4 7.1 0.56 -3.1 x x x x x x x x x
 
99 Cameron Jerome 74 8 7.7 1.04 0.3 x x x x x x x x x
 
100 Gervinho 74 9 9.6 0.94 -0.6 x x x x x x x x x
 
101 Darren Bent 73 12 10.9 1.10 1.1 x x x x x x x x x
 
102 Djibril Cissé 71 9 7.3 1.23 1.7 x x x x x x x x x
 
103 Steve Morison 70 10 6.5 1.55 3.5 x x x x x x x x x
 
104 Nicklas Bendtner 70 9 8.0 1.13 1.0 x x x x x x x x x
 
105 Branislav Ivanovic 68 9 8.4 1.07 0.6 x x x x x x x x x
 
106 Damien Duff 67 5 5.4 0.92 -0.4 x x x x x x x x x
 
107 Jamie Mackie 65 9 8.3 1.09 0.7 x x x x x x x x x
 
108 Yakubu 65 13 8.7 1.49 4.3 x x x x x x x x x
 
109 Lukas Podolski 64 12 6.4 1.87 5.6 x x x x x x x x x
 
110 Kenwyne Jones 63 4 7.6 0.52 -3.6 x x x x x x x x x
 
111 Angel Rangel 62 3 5.8 0.51 -2.8 x x x x x x x x x
 
112 Wayne Routledge 59 6 5.7 1.05 0.3 x x x x x x x x x
 
113 Aaron Lennon 59 9 6.0 1.50 3.0 x x x x x x x x x
 
114 Tomas Rosicky 58 3 5.4 0.55 -2.4 x x x x x x x x x
 
115 Raheem Sterling 58 5 6.5 0.77 -1.5 x x x x x x x x x
 
116 Jonas Olsson 58 3 6.6 0.46 -3.6 x x x x x x x x x
 
117 Patrice Evra 57 5 5.5 0.91 -0.5 x x x x x x x x x
 
118 Álvaro Negredo 56 8 7.7 1.04 0.3 x x x x x x x x x
 
119 Gareth McAuley 56 7 8.2 0.85 -1.2 x x x x x x x x x
 
120 Adam Le Fondre 55 9 6.5 1.38 2.5 x x x x x x x x x
 
121 Yoan Gouffran 55 9 6.9 1.31 2.1 x x x x x x x x x
 
122 Thomas Vermaelen 54 6 5.9 1.02 0.1 x x x x x x x x x
 
123 Marc-Antoine Fortuné 53 4 5.4 0.75 -1.4 x x x x x x x x x
 
124 Ryan Shawcross 53 4 6.3 0.63 -2.3 x x x x x x x x x
 
125 Pavel Pogrebnyak 52 11 6.5 1.69 4.5 x x x x x x x x x
 
126 Ashley Williams 51 1 6.4 0.16 -5.4 x x x x x x x x x
 
127 Phil Jagielka 51 4 6.6 0.61 -2.6 x x x x x x x x x
 
128 Fraizer Campbell 50 7 5.2 1.35 1.8 x x x x x x x x x
 
129 Paulinho 50 2 5.3 0.38 -3.3 x x x x x x x x x
 
130 Martin Skrtel 50 6 5.5 1.10 0.5 x x x x x x x x x
 
131 Russell Martin 50 5 5.5 0.92 -0.5 x x x x x x x x x