Sunday, June 25, 2017

Thoughts on Stats and How to Press

The Rangers Report ran an excellent article a few weeks back that focused on how simple stats could be used by teams to press more effectively. One of the main ideas was that by looking at how many times a player loses possession divided by how many times he had possession overall (particularly in the defensive third), we can see how likely he is to lose the ball, and therefore whether it makes sense to press him. It's an intuitive concept, one that I especially like since it fits in very neatly with my own work on Usage Rates. However, I think it is an idea that needs to be more thoroughly explored, and I'd like to offer my own slightly differing viewpoint on the subject.

First of all, the article in question focused on a sample of just four games. While I don't have the data split into thirds of the pitch, I do have all of the relevant stats for the full field for all Premier League teams from the 16-17 season (the raw data is available in my Usage Rate info here). I would also point out that Statszone uses the location of the end of the pass to determine which third it is in, not the beginning. Since from a pressing perspective the priority is where the ball is passed from, the classification Statszone provides is not as helpful as it could be for this purpose, particularly for a team that hits as many long balls as West Brom. The full field data for West Brom in 16-17 is below:

Player Minutes P90 Possessions Loss of Possession % Last 4 Minutes Last 4 possessions p90 Last 4 Loss of Possession %
Dawson 3276 28.3 37% 360 24.0 34%
McAuley 3140 22.2 35% 153 12.9 32%
Rondon 2892 26.1 32% 340 28.3 25%
Wilson 255 27.2 31% 255 27.2 31%
Nyom 2637 25.5 30% 310 26.4 22%
McClean 1480 23.3 29% 199 27.1 35%
Brunt 2477 33.1 25% 360 27.3 25%
Robson-Kanu 751 23.2 25% 31 17.4 17%
Fletcher 3231 34.7 23% 360 28.5 27%
Livermore 1301 36.9 23% 322 32.1 24%
Chadli 2135 30.2 20% 122 31.7 19%
Evans 2637 31.2 19% 320 26.7 15%
Morrison 1751 40.3 19% 146 37.0 13%
Yacob 2422 34.8 18% 227 32.5 12%
Field 291 32.2 18% 103 27.1 32%

When the sample is broadened, it becomes clear it's a bit unfair to pick on Marc Wilson too much. Those four games were the only PL games he played in all year and he appears to have been played at LB (or left CB in a three) rather than his natural position of CB, which is compounded by the fact he is primarily right-footed (he's listed as both-footed via Transfermarkt, but according to James Yorke he shoots more often with his right so I'm calling him right-footed). While that in itself may have been a good reason to press him in those games, it doesn't help much in finding a long-term strategy. Overall though, we can see the pattern generally holds: the correlation between a player's possessions per 90 in the sample versus the full season has an R-squared of .61, while the correlation of the Loss of Possession % has an R-squared of .43 (this also improves when you add a minutes requirement to the sample, as Leko and Field screw things up).

More fundamentally though, there is an important question here: what are the statistical attributes of players an opposing coach should attempt to press directly? Part of this answer depends on the aim of pressing obviously: if you are attempting to intercept the ball in your attacking third your answer may differ from someone who is simply aiming to coax the other team into giving back possession. Still, I think this passage from Marti Perarnau's piece on pressing demonstrates the general things we should be looking for:

The ball is much easier to take from an opponent who controls it poorly. A team can collectively press the ball at the moment it’s miscontrolled because it would take time to re-establish control of the ball. This plays a part in the “opponent’s ability” as well. If the player is very poor at making decisions and controlling the ball it would be logical to put that player under immense pressure as soon as he’s about to receive it. Most players are taught to press the opponent “as the ball is traveling” because the scene cannot change dramatically within the time the presser leaves his position as the ball is moving between players.

The ball cannot dynamically change directions in the middle of its route between players (unless there is some crazy spin on the ball, which would be visible and anticipated by the players) so it is an optimal time to press the destination point of the ball. If the presser decided to leave his position while it is under the control of the opponent player (without the following layers of the press to protect the vacated space and cover him) then the ball could change direction quite easily as the opponent can simply dribble and exploit the movement of the presser.

The Rangers Report article makes the case that it is the rate at which a player loses possession (particularly in their own defensive third) that should indicate who to press. But the Possessions Lost in the article's calculation are primarily misplaced passes, by an order of magnitude over everything else. In other words, the Loss of Possession % is not necessarily showing a lack of ability on the ball (which Perarnau would suggest is the key to a good target of pressure), but rather is showing a lack of passing ability. To me, that means the Loss of Possession % as used there does not indicate who to press, it indicates when to press: i.e. the passes from a player with a high Loss of Possession % should serve as a trigger to initiate the press since they are more likely to be lost. As for who to press, we would need to find players who are "poor at making decisions and controlling the ball", ideally those who are targets of poor passers. This would suggest looking at players with more times dispossessed, more unsuccessful touches (both of which I should point out are unavailable at either Statszone where the author pulled his data or Squawka where I got mine, but are shown on Whoscored), and poor dribble/take-on success.

So what does looking at those stats tell us? I added Unsuccessful Touches and Dispossessed to the Possessions calculation, and looked at what percentage of Possessions result in an on-ball turnover (TO) for all defenders and central midfielders. The results are below:

Player Minutes Unsuccessful Touches p90 Dispossessed p90 Unsuccessful Take-Ons p90 TOs p90 Possessions p90 TO%
Galloway 248 2.2 0.7 0.0 2.9 9.1 32%
Morrison 1740 1.8 1.8 0.4 4.0 13.8 29%
Fletcher 3235 1.3 1.2 0.2 2.7 11.8 23%
Nyom 2639 1 0.6 0.4 2.0 9.8 21%
Gardner 213 1.3 1.3 0.8 3.4 18.7 18%
Livermore 1303 1.2 0.6 0.3 2.1 12.0 17%
Yacob 2418 0.7 0.5 0.1 1.3 8.2 16%
Brunt 2479 0.8 0.6 0.1 1.5 12.7 12%
Dawson 3278 0.8 0.4 0.2 1.4 12.9 11%
Evans 2638 0.4 0.3 0.1 0.8 7.3 11%
Wilson 256 0.4 0.4 0.0 0.8 10.3 8%
McAuley 3143 0.2 0.1 0.1 0.4 8.6 5%
Olsson 591 0.2 0 0.0 0.2 9.6 2%

As you can see, it seems to be ruled primarily by position. The central midfielders and fullbacks are higher, then the centerbacks, which is basically what you'd expect. As such, I would probably give the CBs space, make sure my defense is in good position, and start my press upon passes into midfield or to the fullback. It should be noted that a lot of pressing systems attempt to force passes to the sideline, as the sideline reduces options for the man in possession (sidenote: I had a FIFA flashback typing that sentence), so this would seem to confirm that's an effective strategy against West Brom. Passes from Wilson or McAuley (who have a lot of passing errors) to Nyom or Galloway (who struggle on the ball) seem like an ideal time to press to me.

It's important to note that I have not done any league-wide comparisons on this, so I don't know if West Brom's statistics are notably different from the rest of the league. It may be that their CBs actually are more prone to on-ball turnovers than those of other teams, and that it would make sense to press them as a result. We also don't have great metrics to judge the effectiveness of pressing, so it would be difficult to judge how following these recommendations would help or not, and how that impacted a team's overall performance. Still, I think this is definitely the right area to be looking, as finding the players likely to lose the ball can only help you win it back.

(Shoutout to Nico Morales for his feedback on the tactics side of things.)