So I had originally intended to put together a blog post about “every team’s best transfer option” but programming headaches and my day job got in the way. But a day late, I finally got the script right, and I’ll have a lot of interesting data-driven posts to share with you all.
Today’s post looks at “room for improvement.” What I did calculate a Random Forest model to predict each team’s expected points this season. Then I took each player out of Arsenal’s lineup and substituted every player in my database in for each player in Arsenal’s lineup, using the same Random Forest to recalculate the expected points. Finally, I calculated the proportion of players that would give Arsenal more points, and graphed in here:
So to walk through an example, the first player I looked at was Laurent Koscielny (because I started with defenders). I calculated Arsenal’s expected points (around 80.5 in the Random Forest model), and then replaced Koscielny with Abdoulaye Ba, the first player alphabetically in my database of European defenders1. I re-ran the RandomForest with the rest of Arsenal’s 10 outfield players and Ba in for Koscielny, and predicted they would only earn 73.5 points with him instead of Koscielny. I then repeated this for Adriano instead of Koscielny, and Arsenal would earn 75 points with him. Jordi Alba would be 79 points, etc. through the rest of the list.2 I then calculated the percentage of players in the whole dataset who replaced Koscielny who would earn Arsenal more than 80.5 points, and saved that number (5.7%, or 0.057 as a proportion). I repeated this for the rest of Arsenal’s players to get the same value for the remaining 10 outfield players.3
The big finding here is that Arsenal doesn’t have a lot of opportunities to improve. One can assume that most of the 5% of the players who can improve upon Koscielny and Bellerin are at top clubs already and aren’t available for transfer, so there aren’t a lot of options. I’ve always been a big supporter of Wenger’s “only buy the right player” strategy, and this lends some empirical support to that.4
The other finding is that Mesut Ozil dramatically underperformed last year, and needs to step up his game significantly. I haven’t seen the other top teams, but it’s odd that a team like Arsenal could keep a player who was only in the ~60th percentile at his position last year.
More teams to come (Aston Villa’s halfway there), and a Points Above Replacement (PAR) measure is coming relatively soon. I might put together a draft of something tomorrow, but I need to think the model through a lot more.
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- I removed all players with fewer than 10 games played – the first iteration didn’t do this and had some striker who had played in 1 game for 15 minutes and got two shots off in those 15 minutes bumping Arsenal up to 95 points ↩
- Unfortunately my data doesn’t have information on specific positions outside defender, midfielder, and forward so there are some weird left-back for center-back substitutions and even more in the midfield ↩
- I’m working on this method for all 20 EPL teams, but each team takes about 5-6 hours to run on my computer so it will take a week or two to get the data done. Hopefully closer to a week, but time and energy constraints might get in the way for all you Watford fans who read this blog. ↩
- I don’t have the full list, but I know Cavani is on the list of strikers who would be an improvement over Giroud so he was on the right track there, even if he was a bit tentative. ↩