In our NHL analysis, we spend a lot of time looking at shot ratios (Corsi, Fenwick, etc…) as a proxy for puck possession or offensive zone time. These are fairly consistent quantities from year-to-year, and teams retain nearly 60% of their performance:
Compare that to ball possession in soccer:
EPL teams have virtually zero year-to-year variation in ball possession. If you know how a team did last year, then you've got a pretty good idea of how they'll do next year. There are other elements – goaltender talent, shooting talent, and a whole lot of luck – but teams are essentially stuck where they are. Here are teams that been in the EPL over the last four seasons:
| Avg | >40 | >45 | >50 | >55 | >60 | Max-Min | |
| Chelsea | 63.86 | 4 | 4 | 4 | 4 | 4 | 2.17 |
| Man Utd | 61.73 | 4 | 4 | 4 | 4 | 4 | 2.92 |
| Arsenal | 61.62 | 4 | 4 | 4 | 4 | 4 | 2.30 |
| Liverpool | 61.09 | 4 | 4 | 4 | 4 | 2 | 4.13 |
| Spurs | 53.66 | 4 | 4 | 4 | 1 | 0 | 3.36 |
| Man City | 49.88 | 4 | 3 | 3 | 0 | 0 | 9.53 |
| Everton | 49.84 | 4 | 4 | 2 | 0 | 0 | 6.50 |
| West Ham | 49.49 | 4 | 4 | 1 | 0 | 0 | 4.87 |
| Blackburn | 48.09 | 4 | 3 | 2 | 0 | 0 | 11.73 |
| Fulham | 47.78 | 4 | 3 | 1 | 0 | 0 | 8.16 |
| Man City | 47.43 | 4 | 4 | 0 | 0 | 0 | 3.24 |
| Aston Villa | 46.85 | 4 | 4 | 0 | 0 | 0 | 4.39 |
| Wigan | 46.46 | 4 | 2 | 1 | 0 | 0 | 10.68 |
| Bolton | 41.28 | 2 | 1 | 0 | 0 | 0 | 13.10 |
Obviously there are structural reasons why Chelsea, Man United, Arsenal and Liverpool dominate play, but most teams are trapped in a very small range of performance regardless of year-to-year changes they make to their teams. It's unfortunate that we don't have passing and possession data for the NHL – we could see if this is an equivalently-persistent skill in hockey. It seems unlikely, but having pass information as opposed to just shots as we do today would leave us much less at the mercy of luck.
