Dienstag, 5. Mai 2015

Reader's Notice: Publication of Goalimpact

We are happy to announce that the results of the new Goalimpact algorithm are published for the Premier League and for the Bundesliga on our partner sides

Especially on PremierInsider there might still be some missing charts. We are working on it.

Have fun while browsing!

Montag, 4. Mai 2015

How good is Red Bull's Stefan Ilsanker?

One of the most surprising results of the latest top-50 list of football players, was the high ranking of Stefan Ilsanker. He was rated despite playing in the mediocre Austrian Bundesliga, albeit at the league dominating Red Bull Salzburg. So the question of this post is: How much Red Bull is in Ilsanker's Goalimpact?

One way to look at Goalimpact, is to think of it how good a team plays with a player compared to the team without. In this case, the Goalimpact is calculated by the difference in goals scored, the difference in goals conceded and the average Goalimpact of the replacements. This is only simplified, because Goalimpact corrects for other factors such as the home field advantage, too, but it is a good starting point that is reasonably good if calculated over many games.

The following chart shows Red Bull Salzburg's goal difference with and without Ilsanker starting from July 2012 until today.


As you can see, with Ilsanker Salzburg had an average goal difference of 1.96. Without him only of 1.56. So there is a strong improvement of results of 0.4 goal difference per game if Ilsanker plays. To put this in perspective. If Red Bull was to play all league matches with Ilsanker, Red Bull would be expected to end up with a total goal difference of +71. A season without him would be less dominating and ending with a goal difference of 'only' +56.

This is only an indication that Ilsanker does improve the team significantly. As argued before, there might be other factors that correlate with Ilsanker playing that create this improvement in goal difference. One example would, e.g., be the quality of opposition. If Ilsanker would only play against bad opposition then Salzburg's goal difference would be good because of that rather than Ilsanker's brilliance. However, given that Goalimpact corrects for this it may not be the case here.

Another caveat of our analysis is that we showed that Ilsanker adds goal difference to the team, but maybe the team as such is overvalued? Let's perform another test trying to address both points. If we redo the analysis, but only on the UEFA games of Salzburg, we can see if they are strong there, too. Additionally, we can assume that Salzburg will play its best players in European matches.

In total, we have 28 matches of Salzburg on European level since July 2012. In the 2036 minutes with Ilsanker, Salzburg had a goal difference of +25 (51 to 26). In the 568 minutes without him it was +5 (12:7). In goal difference per minute this makes 1.14 with him and 0.82 without him. Even in this subsample he added 0.32 of goal difference. Slightly less than on the whole sample, but still a handsome adder.

Since Salzburg's goal difference is positive even in the European matches, it looks like they are not per se overvalued. However, they didn't meet very big teams too often, so it is difficult to tell for sure, but they met

  • Fenerbahce: one 1:1 draw and a  1:3 defeat. Both with Ilsanker
  • AFC Ajax: two victories. 3:0 and 3:1. Both with Ilsanker
  • FC Basel: one 0:0 draw and a 1:2 defeat. Both with Ilsanker
  • Dinamo Zagreb: two victories. 4:2 and 5:1. Both with Ilsanker
  • Celtic FC: one 2:2 draw and a 3:1 win. Both with Ilsanker 
  • Villarreal CF: two defeats. A 1:2 on road with Ilsanker and a 1:3 without at home.

All defeats came against teams that were lower ranked than they. This indicates that there might be an overvaluation of Red Bull Salzburg, but we are talking very small N now. Other team ratings, rank Salzburg considerably lower. So there is an indication, that Salzburg is overvalued and, in turn, Ilsanker is. but the uncertainty is significant. However, there is strong evidence that Ilsanker is pivotal to Salzburg's performance and hence sticks out in the team.

Let's look forward to the next European season and Salzburg's next try to play Champions League. We will get a clearer view then on where they stand. Hopefully they'll play with Ilsanker.

Samstag, 2. Mai 2015

How fast does Goalimpact converge?

If you saw only few games of a player, it is hard to tell if he is good or not. If you saw all games of a player in his career after he retires, you will have a pretty clear picture if he was any good. In this article we test after how many games Goalimpact is giving a good estimate of the player's ability.

Before we can test the algorithm, we need an estimate of the true ability of the player. We do this, by restricting the sample on players that finished their career already. For those we proxy the true skill by their career end Peak Goalimpact. To eliminate players where this isn't a good proxy for true skill, we further restrict the sample to players that had at least 20000 minutes of playing time at the end of the career. The average player remaining in the sample had 32,000 minutes playing time at career end.

Now we will compare the predicted PeakGI after a limited number of minutes, early in the career, with the career end PeakGI. We quantify the quality of prediction by R² in the following table.

Minutes Field Player Goalkeeper
1000 8.30% 5.05%
2000 15.20% 8.87%
4000 28.70% 19.99%
8000 50.87% 40.88%

So after 1000 minutes of a field player, slightly more than ten games, the then estimated PeakGI explains 8.3% of the variance of the career end PeakGI. That is still a pretty uncertain prediction, but given that this is based on only 1000 minutes, the information content is surprisingly high. Goalimpact actually does separate good and less good players after just 10 games to some extend.

After twice as many minutes the explained variance is already more than 15%. This is a very good result because 2000 minutes is just a bot more than half a season of input. So very early in a players career Goalimpact shows his discriminatory power.

Another doubling of the number of observed minutes and the R² raises to nearly 30%. And it becomes more than 50% after just 8000 minutes or about two seasons worth of observations. Many players will not be even 22 by then. In fact, if we further restrict the sample to players that reached 8000 minutes of observation before turning 22 years, the R² is still an outstanding 34%.


For goalkeepers the results are consistently lower, but they stay in the same order of magnitude. The prediction quality for goalkeepers with 8000 minutes of playing time is still a very good 40%.

Summary

We showed that the PeakGI early in the career is predictor for the future career path of the player. After as few as 10 games, we already found some predictive power. After 8000 minutes, a large part of the true skill difference between players has been identified - even for very young players. For goalkeepers results are consistently lower, but in the same order of magnitude.

Samstag, 25. April 2015

Young Talent Challenge

"Predictions are difficult, especially about the future." coined Nils Bohr once famously. The equivalent in football is the scouting of future top players. Goalimpact is designed to predict future performance - as opposed to describing past performance. Hence we would assume that players identified as future top players by Goalimpact have a higher than average chance of indeed become a football star.

The holy grail of identifying talents is identifying them before everybody did and hence before they become expensive. In this post, we want to start a series of tracking young players on their quest to become a top player. Some of the players are known talents, some are yet to be uncovered. As proxy for if a player is known or not, we use the market values as published by Transfermarkt. Any player with a value above 1.5M€ is declared to be known (although maybe still very cheap), any player below or equal to that number is defined as unknown. We will not publish the names of unknown players, but we assign numbers to them to be able to track them in future post.

Ok, here is the list of the most talented players according to Goalimpact that are yet to turn 20. If a player is shown as 20, then this is a rounded number.

Player Transfermarkt Goalimpact PeakGI Age
Julian Brandt, Bayer Leverkusen 5,000,000 € 132.5 171.3 19
Max Meyer, FC Schalke 04 15,000,000 € 122.9 154.8 20
Anonymous Talent 1 300,000 € 99.6 145.9 18
Anonymous Talent 2 1,000,000 € 96.9 145.3 18
Anonymous Talent 3 1,000,000 € 98.4 144.8 18
Richairo Živković, Jong Ajax 4,000,000 € 101.7 144.3 19
Rubén Neves, FC Porto 5,000,000 € 95.7 144.0 18
Anonymous Talent 4 500,000 € 96.2 143.5 18
Yoeri Tielemans, RSC Anderlecht 12,000,000 € 95.4 143.3 18
Andrija Živković, Partizan 3,000,000 € 101.9 142.8 19
Anonymous Talent 5 350,000 € 102.2 142.2 19
Anonymous Talent 6 50,000 € 111.5 141.8 20
Anonymous Talent 7 500,000 € 107.0 141.5 19
Breel-Donald Embolo, FC Basel 8,000,000 € 93.9 141.4 18
Anonymous Talent 8 500,000 € 97.8 141.3 19
Anonymous Talent 9 750,000 € 103.0 141.1 19
Anonymous Talent 10 200,000 € 107.3 140.7 19
Anonymous Talent 11 50,000 € 92.0 140.4 18
Anonymous Talent 12 125,000 € 96.9 140.1 19
Anonymous Talent 13 125,000 € 101.5 139.7 19
Anonymous Talent 14 250,000 € 105.1 139.4 19
Teddy Bishop, Ipswich Town 2,500,000 € 98.3 139.3 19
Pierre-Emile Højbjerg, FC Augsburg 4,000,000 € 108.5 139.0 20
Anonymous Talent 15 1,000,000 € 91.6 138.8 18
Anonymous Talent 16 0 € 95.7 138.8 19

Most staggering result is Julian Brandt that is rated as future football god. Between him and the May Meyer on number two is already a huge gap. Given that talent, I wonder how long he will be playing at Leverkusen. Big teams must be queuing already to get him on board. On rank three is already an unknown player. He is playing in the second division of a non Top5 country.

We will post an update of this list maybe once per quarter tracking the players. Let's see how far we get. Buying all unknown players costs us 6.7M€. Let's see how this value changes over time.


Julian Brandt still just a demigod, but soon will be grown up.

Sonntag, 12. April 2015

How to read a Goalimpact Chart?

Following the updated algorithm and the latest Top-50 players list, we received a lot of questions to individual players and on how to read the chart. Let's answer both in one post.

Zlatan Ibrahimovic

Easily the most requested player following the Top50 post. Probably, because he was not on the list. That doesn't mean that Goalimpact isn't rating him, he is just not in the Top50. Main reason is that he dropped out of the list due to aging. Here is the chart.


The thick line shows the Goalimpact at that time. This is the original estimate not using any future games. Clearly, with hindsight, we may give him another rating, because his team outperformed or underperformed original expectations. The expectations on how good a Zlatan will be in future, are derived from the Peak Goalimpact (thin dashed line) and the aging curve of field players.

If the team results of all of Zlatan's games are better than expected by his Goalimpact, the PeakGI line will raise. It will do that whenever the player overachieves original expectations, independent if he passed his peak already or not. In the recent year, for example, his PeakGI raised because his performance dropped less than we would have expected given the typical aging effect in football. Since 2011, the peak rose nearly 15 points. Without that raise, his Goalimpact would have been 15 points lower than it is today, so 140 instead of 154 points.

To summarize, Zlatan is a world-class player that is gradually weakening due to aging. But this aging process is much slower with him than with the average football player. His current performance still is outstanding at 154.

Theo Walcott

The high rating of Theo Walcott raised some virtual eyebrows on Twitter. So this is how his chart looks like:

Theo was rated "world-class one day" since he was very young. Given his young age, he was still rated as "ok for Premier League, but not outstanding". But this is his actual performance and not his "potential performance" or "talent", which both were rated as very high. Walcott delivered as expected until 2012. Therefore his PeakGI was more or less stable and the Goalimpact rose along expectations given the aging curve. From 2012 on, the team strongly outperformed expectations when he played and hence his PeakGI rose consistently until it reached a level of about 190 when match outcome and Goalimpact were in agreement.

Back to the Twitter question of why Walcott is rated that good: Because the team consistently outperformed prior (already very high) expectations with him on the field.

Cristiano Ronaldo

"Cristiano Ronaldo is only 21st? Lol, he would walk into any team of the world" was a typical comment. We understand the critic because he is one of the best players of the world (also in according to Goalimpact) and many see him as the best (also Goalimpact before the change of the algorithm). So why did he drop in the new algorithm?


He actually didn't drop in the new algorithm, he maintained the world-class level he obtained in his Manchester United times seamlessly at Real Madrid. Real's performance with him on the field was fully in line with these very high expectations and thus there was no need to raise or lower the rating.

"Yes, ok, but he was rated higher in the old version". True, the reason is that Goalimpact adjusts expectations for all players of the field. The new version rates Karim Benzema higher than the old. This, in turn, raises the needed outcome for all his team mates to increase their respective scores. Hence, with Benzema rated higher, his team mates including Ronaldo were rated lower. That said, Ronaldo is rated as absolute world-class and would also according to Goalimpact walk easily into any team of the world.

Karim Benzema

So? And why is Benzema that good?


We can't answer that. Goalimpact just measures how good a team plays with the player on the field. It can't tell why a player is adding to the team success., because it does not even look on what he is doing. It just relates the team success to the player being present on the field. If we define a "good player" as a player that make his team have a very good goal difference, then Benzema is extraordinarily good.


Top50 Football Players - April 2015 Edition

It is nearly one year ago that we published the last "Top50" list and hence we should expect many changes. But as we also just updated the algorithm there are even more changes and the list is a bit difficult to digest. Here are the main findings so far

  • Most players on this list have been rated high one year ago in the previous version of the algorithm already. On rank 17, Georginio Wijnaldum is the first player that hasn't been Top100 before already.
  • Compared to the old algorithm, the players seem distribute now stronger over leagues and teams. Previously, it was very much focused on the big teams.
  • In the old list, there has been a few players with exceptionally high Goalimpact. The new list doesn't have skill gaps apart of the one that separates Thomas Müller from the rest of the world. This results in many players having a higher Goalimpact and only few players a lower in this list. Overall, the rating scale didn't change. The average is still 100.
  • Despite Austrian leagues having suffered a lot from the new league adjust in the algorithm, two Red Bull Salzburg players made it to this list. Three, if you consider Kampl's breed.
  • Can somebody please buy Vladimir Stojković for 1M€ and let him play in a league where we can watch him to access if he is still really that good?
  • Former Top50-Players no longer that dropped out of the Top100. That is partly due to the change in algorithm, partly due to age or lower performance: 
    • Philipp Lahm (156.94)
    • Xabi Alonso (156.52)
    • Vincent Kompany (154.17)
    • Gianluigi Buffon (153.86)
    • Scott Brown (152.23)
    • John Terry (150.84)
    • Patrice Evra (145.12)
    • Wesley Sneijder (142.09)
    • Ashley Cole (141.81)

Where curious to hear your thoughts.

Rank Player Team Goalimpact Age PeakGI Nationality Previous
Rank
GI Diff
1 Thomas Müller Bayern München 217.30 25.58 218.49 Deutschland 8 +44
2 Mesut Özil Arsenal FC 201.95 26.50 202.86 Deutschland 15 +38
3 Karim Benzema Real Madrid 199.45 27.25 201.99 Frankreich 18 +41
4 Lionel Messi FC Barcelona 199.33 27.75 202.83 Argentinien 4 +17
5 Robert Lewandowski Bayern München 198.36 26.58 199.57 Polen 37 +53
6 Cesc Fàbregas Chelsea FC 196.06 27.92 199.83 Spanien 5 +17
7 Theo Walcott Arsenal FC 192.75 26.08 192.83 England 57 +52
8 Manuel Neuer Bayern München 191.69 29.00 192.79 Deutschland 11 +22
9 Luiz Gustavo VfL Wolfsburg 190.94 27.67 194.28 Brasilien 55 +49
10 Marcelo Real Madrid 189.24 26.92 190.99 Brasilien 29 +39
11 Busquets FC Barcelona 187.84 26.67 189.24 Spanien 26 +35
12 Fraser Forster Southampton FC 185.76 27.00 187.97 England 65 +46
13 Pedro FC Barcelona 185.10 27.67 188.41 Spanien 49 +42
14 Mario Götze Bayern München 183.74 22.83 193.00 Deutschland 36 +37
15 Toby Alderweireld Southampton FC 183.06 26.08 183.22 Belgien 54 +41
16 Piqué FC Barcelona 182.54 28.17 186.72 Spanien 22 +27
17 Georginio Wijnaldum PSV Eindhoven 182.11 24.42 186.33 Niederlande 185 +53
18 Kevin Kampl Borussia Dortmund 182.06 24.50 186.04 Slowenien 70 +43
19 Stefan Ilsanker RB Salzburg 181.49 25.83 181.83 Österreich 106 +47
20 Jeremain Lens Dinamo Kiev 181.21 27.33 183.88 Niederlande 66 +41
21 Cristiano Ronaldo Real Madrid 180.74 30.17 188.42 Portugal 1 -16
22 Gaël Clichy Manchester City 180.69 29.67 187.05 Frankreich 21 +25
23 Andriy Pyatov Shakhtar Donetsk 179.64 30.75 179.77 Ukraine 69 +40
24 Antonio Valencia Manchester United 178.31 29.67 184.63 Ecuador 310 +55
25 Wayne Rooney Manchester United 178.19 29.42 184.20 England 9 +6
26 Mats Hummels Borussia Dortmund 178.15 26.25 178.73 Deutschland 23 +23
27 Andreas Ulmer RB Salzburg 178.01 29.42 183.99 Österreich 61 +37
28 Ángel Di María Manchester United 177.48 27.17 179.70 Argentinien 35 +30
29 Bastian Schweinsteiger Bayern München 177.41 30.67 188.08 Deutschland 3 -6
30 Eden Hazard Chelsea FC 177.36 24.25 181.99 Belgien 122 +45
31 Arturo Vidal Juventus 177.05 27.83 180.73 Chile 48 +34
32 Nani Sporting CP 176.99 28.33 181.47 Portugal 117 +44
33 Arjen Robben Bayern München 176.72 31.17 190.43 Niederlande 19 +19
34 Petr Čech Chelsea FC 175.97 32.83 179.86 Tschechien 17 +17
35 Daniel Agger Bröndby IF 175.88 30.33 184.44 Dänemark 182 +47
36 Gregory van der Wiel Paris Saint-Germain 175.85 27.17 178.14 Niederlande 27 +24
37 Jonny Evans Manchester United 175.51 27.25 177.97 Nordirland 213 +48
38 Douglas Dinamo Moskva 175.01 27.25 177.42 Niederlande 125 +43
39 Toni Kroos Real Madrid 174.34 25.25 176.33 Deutschland 68 +35
40 Salomon Kalou Hertha BSC 172.43 29.67 178.75 Elfenbeinküste 50 +30
41 Joe Hart Manchester City 172.10 27.92 173.80 England 132 +40
42 Vladimir Stojković Maccabi Haifa 171.99 31.67 173.41 Serbien 283 +48
43 Willian Chelsea FC 171.32 26.67 172.58 Brasilien 58 +30
44 Kwadwo Asamoah Juventus 170.90 26.33 171.51 Ghana 139 +40
45 Ezequiel Garay Zenit St. Petersburg 170.88 28.50 175.51 Argentinien 124 +39
46 Alexis Sánchez Arsenal FC 170.69 26.25 171.24 Chile 75 +33
47 Alex Song West Ham United 170.69 27.58 173.77 Kamerun 64 +30
48 Jérôme Boateng Bayern München 169.29 26.58 170.43 Deutschland 33 +20
49 Danijel Subašić AS Monaco 169.06 30.42 169.38 Kroatien 340 +47
50 Javier Mascherano FC Barcelona 168.94 30.83 180.47 Argentinien 14 +4


Georginio Wijnaldum picked up in summer 2011 when he
moved to PSV. Since then, it has been a continues increase
to world-class level. Not only in Goalimpact terms, but also
shown by a third place in the World Cup and the this years' 
victory of the Dutch league.

Donnerstag, 9. April 2015

New Algorithm Released

It has been silent on the blog lately, but it wasn't a quite time. We have been busy putting together a new version of the algorithm that promises a better discriminatory power between good and better players. The main change was to improve the comparability of the Goalimpact values between the different leagues.

What was the issue?

Previously, some remote leagues such as New Zealand or Austrian amateur leagues weren't closely enough related to core leagues to have them rated correctly compared to each other. This resulted in overrated players from those leagues showing up in some top player lists that clearly didn't belong there. This could be solved easily by filtering all players in those leagues.

More worrying, however, was if any of those player did move to another league. He will then show up in reports with the high rating and it will be difficult in all cases if the rating was just earned by some season in a missvalued league.

The solution

More care is now taken to ensure that all information on the relative value of the leagues is properly reflected in the player ratings. This is done in a moderate, very careful way. In particular, there is no "Leaguefactor" and hence we do not expect any jumps of rating if a player moves from one league to another. Also promoted teams are expected to be better ranked compared to the other teams. In the old algorithm they tended to be slightly overvalued.

The current status

The new model is released and the database is fully updated. Any results we publish from now on will be based only on the new version. The update is brand new and hence we didn't have the time to check all results so far. So in the coming days and weeks we will post some slicing of the data to get and give a feeling of the new model. While we are confident that the new model is an improvement over the old, it is still an exiting journey into the unknown for us.

As a first glimpse, here are the best teams according to the new algorithm. The values are minute-weighted averages of all players that played that season for the team. Hence it may not entirely reflect the current team composition. E.g. Dortmund now is probably a bit better due to the signing of Kevin Kampl in the Winter break and Red Bull Salzburg accordingly a bit weaker.

Team Goalimpact Peak GI Min Max
Bayern München 168.1 176.8 108.1 217.3
FC Barcelona 159.0 167.1 108.5 199.3
Real Madrid 157.5 164.1 103.6 199.5
Manchester City 151.1 159.1 97.3 180.7
Chelsea FC 148.5 158.5 96.7 196.1
Manchester United 143.9 153.6 73.8 178.3
Borussia Dortmund 142.3 147.0 78.0 182.1
Paris Saint-Germain 140.2 151.2 76.8 175.9
Arsenal FC 139.3 148.3 83.8 202.0
Juventus 137.7 148.9 85.6 177.1
VfL Wolfsburg 136.6 143.3 103.1 190.9
Atletico Madrid 136.4 146.4 103.5 160.1
Zenit St. Petersburg 135.0 143.4 78.2 170.9
Dinamo Kiev 132.7 138.1 87.0 181.2
Southampton FC 131.5 137.8 97.8 185.8
Shakhtar Donetsk 130.5 140.6 81.0 179.6
Liverpool FC 130.2 141.8 91.1 165.5
Bor. Mönchengladbach 128.6 136.0 97.4 164.5
FC Porto 128.2 136.2 95.7 166.7
CSKA Moskva 128.2 139.9 83.4 167.1
RB Salzburg 127.6 143.9 89.1 181.5
SL Benfica 127.6 137.6 75.1 147.7
Tottenham Hotspur 126.5 133.8 78.5 159.5
SSC Napoli 125.3 132.1 97.0 159.4
Valencia CF 125.1 133.7 93.3 151.4
AS Monaco 124.6 138.4 96.0 169.1
Bayer Leverkusen 124.5 137.5 96.7 156.3
Dinamo Moskva 124.2 130.7 70.5 175.0
PSV Eindhoven 123.3 136.1 84.5 182.1
ACF Fiorentina 123.2 133.2 89.3 162.5
Everton FC 123.0 136.4 68.3 150.5
Celtic FC 122.6 130.4 76.4 159.6
Sevilla FC 122.3 130.2 91.9 142.3
AS Roma 121.9 135.5 84.2 159.3
Sparta Praha 121.9 131.0 70.5 151.3
FC Schalke 04 121.9 133.4 63.7 162.4
Sporting CP 121.8 128.9 97.7 177.0
AC Milan 121.7 129.6 94.3 166.3
1899 Hoffenheim 121.3 128.2 86.1 153.1
Swansea City 121.0 126.0 76.7 132.2
Viktoria Plzen 120.3 127.4 64.8 144.1
AFC Ajax 119.9 134.7 83.6 152.2
AS Saint-Etienne 119.8 124.9 89.9 142.4
Hamburger SV 119.2 127.7 83.0 143.7
Guangzhou Evergrande 119.0 124.6 90.3 146.0
Hertha BSC 118.8 124.7 93.2 172.4
Inter 118.8 129.0 69.7 151.8
Lazio Roma 118.7 130.5 88.6 154.8
Olympique Lyon 118.4 131.2 84.4 144.3
West Ham United 117.6 124.0 85.2 170.7