Thursday, May 7, 2015

How do soccer players perform after transferring to different teams and leagues?


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High-profile transfers are common in European football - every year, record-breaking transfer fees are offered between teams for the best players in the world. Cristiano Ronaldo, Gareth Bale, Luis Suarez are just three of many superstars in the past few years that have made moves that stunned the football world.

But are these sky-high transfer fees justified for a cross-league move, with no concrete performance metrics in that league to back it up? If the player cannot adjust to the style of play, the rigor, or even the language of the new team, the transfer could render an otherwise superstar player ineffective. After all, players like Juan Sebastian Veron and Karim Benzema have made bold cross-league transfers, only to never capture the form they once had.

By collecting data from soccerway.com, we examine how players perform before and after transferring from teams in the Premier League, Bundesliga, Serie A and La Liga. Through this analysis, we can see patterns in players’ performance as they move from one specific league to another, and use that information to make smarter decisions about cross-league transfers in the future.

Goals Per 90 Minutes

The first metric we use to measure player performance before and after a transfer is goals per 90 minutes. We average together this metric across all players moving from one league to another league before and after the transfer to get a general idea of how that move affects their playing.


(Note - Rows represent the country players are transferring from and columns represent the country players are transferring to)

We see a few interesting observations here:

  1. Not surprisingly, performance of players vary much less when they transfer domestically as opposed to internationally. This supports the notion that players experience markedly different styles in different leagues.
  2. All players appear to have more success in terms of goals scored when they transfer to Germany. This is interesting and also surprising, because Serie A is ranked the worst among the four leagues, according the the latest UEFA Country Coefficients, so it would make sense for players to score more when moving to Serie A from other leagues. That said, Bundesliga players seem to score more goals when they transfer to the English Premier League. This may be due to the quality of players transferring from Bundesliga, suggesting that only the most elite players from Bundesliga join the EPL.
  3. EPL players seem to enjoy plenty of success when they transfer to La Liga, increasing their goal rate by over 60%. Suarez and Bale have scored less since joining La Liga, but apparently these are exceptions. In general, players in EPL enjoy success in all other leagues, justifying the league’s status as one of the more physical and defensively oriented organizations around.
  4. While most players tend to improve their goalscoring prowess after transfers, the most significantly decrease in performance comes when players move from the Bundesliga to La Liga. Spain’s premier division is rated the highest in the UEFA rankings, so it is no surprise that the Bundesliga, which only has one truly elite team in Bayern Munich, struggle to cope with the quality of La Liga.

Minutes Played Per Game


(Note - Rows represent the country players are transferring from and columns represent the country players are transferring to)

This metric calculates the number of minutes of a game a player is on the pitch. We see a few interesting results:

  1. In general, minutes played increases for most players after a transfer. This is an encouraging sign, as it means that teams are making use of their purchases in meaningful ways.
  2. Players seem to find the largest increase in playing time after transferring to the Bundesliga, or when they leave the English Premier League, which may shed some insight on the competitiveness of each league. As mentioned before, EPL has more competitive and higher quality teams, compared to Bundesliga, with the exception of Bayern Munich. To reinforce this observation, Serie A players moving to the Bundesliga increase their playing time increase by over 35% (from 50 to 68 minutes per game). EPL players see their minutes increase by nearly 20% if they move to Serie A (50 to 60 minutes per game).
  3. That said, our dataset contains a significant amount of variance, large enough for all of these changes in minutes played to be deemed “insignificant” by hypothesis testing. To further speculate on any of these insights, more transfer data dating up to at least a decade back would be necessary to make any conclusive observations. (Right now, we have data back to 2011)

The above transfer data seems to support the idea that each league has a different playing style, judging by the comparison between domestic transfers and international transfers. The data also shows certain trends, like players scoring more when they move to Germany, regardless of where they come from. 

However, surprising trends like players not scoring a lot more when moving to Italy even though Serie A is ranked last out of the four leagues should be considered with context - Serie A's richest team is only 8th on the list of most valuable teams in the world, following many other teams in the other three leagues. Thus, the data may show that players moving to Italy do not score more because Serie A teams cannot afford the superstar international transfers that the other three leagues can.

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