Hockey’s Analytics “Aha” moment?
Cam Charron has an interesting post on the state of Hockey Analytics over at The Score and how hockey executives are a step behind the hockey analytics bloggers but I have to disagree with one statement that Charron made.
There’s a reference in the Friedman piece to Craig MacTavish walking around looking for the “Aha!” moment when it comes to hockey analytics. I don’t think MacTavish has realized that half the hockey world is a step ahead of him in that regard. The “Aha!” moment comes when you realize that shots are a hell of a lot more predictive than goals for determining future events. As soon as you realize that hockey is a game between two teams trying to take shots on goal, I think the rest of it falls into place.
The problem with that thinking is that the minute we think hockey is all about shots and not goals the whole system could fall apart. We know that shot quality exists. It’s a fact of life. A 45′ shot is generally not nearly as tough as a 10′ shot. A shot from 20′ after a cross ice pass is more difficult than a shot from 20′ on a two on two rush before the guys turn back for a line change. A screened shot from the point is more difficult than an unscreened shot from the point. Shot quality in that sense exists and is undisputed. The only reason shot analytics work is if over a large enough sample the quality of shots averages out such that the average quality of shot for one team is more or less equal to the average quality of shot for another team. I differ with some the extent that this is the case, but for this discussion I’ll go along with that premise. Now the problem is, when hockey starts to incentivize shots rather than goals I am not certain that that premise will hold up. There are lots of time a player could shoot the puck, but chooses not to because it is not a good scoring chance. If we start rewarding players on the basis of shot totals and that player starts shooting in those bad scoring chance situations the premise by which shot analytics is based on falls apart. Hockey at its core is, and always will be, about scoring goals. The fact that shot differentials correlate highly with winning is an interesting observation, and maybe even a useful one, but to change the focus of the game to shot differentials from goals differentials is not likely a strategy that will work in the long run.
Positive shot differentials is a result of good play and not because a team chose shot differentials as their goal and achieved it. The reality is, to generate positive shot differential you need to:
- When you have control of the puck you generate an offensive opportunity from that puck possession more frequently and you give up control of the puck less frequently.
- When you do not have control of the puck you force the opposing team to give up the puck more frequently and generate an offensive opportunity less frequently.
- You gain possession more frequently than the opposition be that through winning face offs or winning the puck battles after shot attempts.
If you can win the puck battles, give away the puck less frequently and force the opposition to turnover the puck more you should win the shot differential contest. I suspect shot differential is highly correlated with winning because good teams do those three things better than their opposition and not choose to shoot more often than their opposition. We really need metrics to measure those three things but unfortunately we don’t have them. The work being done on carry the puck into the offensive zone vs dumping the puck in is valuable because it hits at the heart of those good attributes (i.e. what is the best way to generate an offensive opportunity when we have possession of the puck).
This isn’t to suggest that looking at shot totals is a bad thing. So long as we live in a world where driving shots is not the primary goal, shots totals can act as a proxy for identifying players who might have some of those other good attributes and since we have no good metric for measuring them. We just have to be careful that we aren’t identifying systems that result in more shots but not more good shots. Again, shots is not the goal, goals are.
Furthermore, it is quite possible that shot differential analytics can result in a value proposition for GMs. In my post last week about the declining predictive value of corsi/fenwick I showed that as sample sizes increase corsi/fenwick does a poorer job of predicting future events at the team level than with smaller sample sizes where as the percentages and goal metrics maintain or improve their predictive value. In that post I deliberately was careful about drawing any conclusions about what it meant because, to be honest, I am not completely sure what it means though I do have a couple of theories. One is that it could mean that corsi/fenwick is largely driven by the depth of the team and for many teams the second and third lines have a fair bit of turnover over the course of 2-3 years (where as elevated shooting percentage or save percentage is largely driven by the elite players who don’t change teams nearly as often). If GMs aren’t evaluating second-tier players using shot differential metrics they may not be replacing the players with similarly talented (shot differential-wise) players. If this were true, it could mean that this is a flaw in current thinking and that a smart GM could exploit this flaw but again by filling his second and third lines with positive shot differential players. This could give his team the depth it needs to win. It is just a theory but one worth exploring more.
In the end though, hockey is all about out scoring the opponent, not out shooting them. Always has been, always will be, and that is the way it should be. Realizing that that shot differentials is highly correlated with winning is not the ‘aha’ moment in the sense that all of hockey should change focus to out shooting over out scoring at the cost of shot quality because that won’t work. The focus always has to be how to generate more shots from good scoring plays, not just generating more shots.