We've spilled what must by now be millions of words at this site on the subject of shot differential (aka – Fenwick or Corsi) and how it's the best predictor of future team success, primarily because it's the most persistent team-level talent we can identify. It's even possible that we've made some headway convincing people that this is true. But for some reason, it's hard for people to make the leap to seeing the other implications of this relationship.
For example, I looked at the Sharks‘ PP and PK performance. By power-play and penalty-kill “efficiency”, the Sharks are #3 and #26 in the league in those two categories, respectively. But is power-play “efficiency” – the percentage of power-plays where a team scores – the best way to determine a team’s true talent on the PP? Let’s look at the most persistent talents on the PP and how much they regress to the mean:
| Even->Odd | |
| PP% | 70 |
| SF/60(5v4) | 44 |
| Shot%(5v4) | 88 |
What I've done here is look at even and odd games over the last four seasons and run a regression between each PP metric in those two bins. Shooting rate – shots for per 60 mins at 5-on-4 – is by far the most persistent talent. PP efficiency and especially shooting percentage regress much more heavily to the mean. A team's shooting percentage in one half of its games has almost no predictive value for the other half of its games.
If we want to know how well a team will do in the future on the PP, what should we look at? Let's regress each of these quantities in the even half of each team's game against their PP efficiency in the odd half of their games:
| Odd PP% | |
| Even PP% | 70 |
| Even SF/60 | 61 |
| Even Shot% | 89 |
Again, this shows the regression to the mean of each variable. Shooting percentage has very little predictive value as we might have expected, but surprisingly (or not surprisingly to those who follow the line of reasoning behind shot differential metrics), the rate at which teams shoot on the PP is a better indicator of their future power-play "efficiency" than their past power-play efficiency.
A number of commenters have expressed discomfort that I don't believe that the team with the highest power-play "efficiency" is the team with the best power-play. But remember that power-play "efficiency" is a flawed measure of talent – it's merely a sample of a team's ability. And ability is not the same thing as performance.
