Yesterday it came across my twitter feed a paper about using regularized logistic regression in estimating player contribution in hockey. I skimmed through the article but not enough to fully understand that article but found some of the conclusions at least mildly interesting. This post is neither a post in support or against the paper but rather a rebuttal to a rebuttal from Eric T at NHLNumbers.com.
To summarize the paper, the authors conducted a goal based analysis to estimate player contribution and to summarize Eric T’s rebuttal, Eric T applauded the effort but suggested a shot based analysis would be more appropriate because that is where ‘modern hockey thought’ currently stands.
I think my biggest concern is that by focusing exclusively on goals, you allow for shooting percentage variance to have a significant impact on a player’s calculated value. Even with four years of data, variance plays a large role in the shooting and save percentages with a given player on the ice.
This is why much of modern hockey analysis starts with shot-based metrics; the shooting percentages introduce a lot of variance which must be accounted for to get a reasonable assessment of talent. If you used shots for your model, I suspect you’d easily identify more than a mere 60 players who have significantly non-zero talent levels — and the model could be further refined from there (e.g. give each shot a weight based on the shooter’s career shooting percentage).
That is in essence Eric T’s argument. Shooting percentages are unreliable so it is better to use a shot based approach (though I find it a little ironic that he then suggest incorporating shooting percentage again).
The “even with four years of data, variance plays a large role in shooting and save percentages with a given player on the ice” is the statement that I have the biggest problem with. It has been shown by myself many times that goal scoring rates are a better predictor of future goal scoring than shot rates are when dealing with multiple seasons of data. Furthermore, any study that uses sufficient amounts of data (either by using multiple seasons of data or by grouping similar players and using their aggregate shooting percentage) has concluded that shot quality (ability to sustain an elevated shooting percentage) exists and is significant. For example, we know that players that get a significant amount of ice time have significantly higher shooting percentages (see here and here and here) and just by looking at list of players sorted by their long-term on-ice shooting percentages we see that good offensive players rise to the top and poor offensive players fall to the bottom (in no way can anyone conclude that that list is random in nature). There is ample evidence to suggest that with 4 years of data goal based metrics should be the preferred tool over shot/possession based metrics.
Eric T brought up Dwayne Roloson, Kent Huskins, Sean O’Donnell, and others as examples of where he feels the evaluation system failed but pointing out a few counter examples is not enough to toss the analysis out completely. There will always be exceptions and outliers when attempting to build an all-encompassing evaluation metric. For the methodology in the paper maybe it is Roloson and Huskins but I can assure you than for any shot based metric it will be Tyler Kennedy and Scott Gomez.
The standard for which an all-encompassing metric should be tested against is not “is it perfect” and if it doesn’t pass that test toss it aside and ignore it forever. These metrics will never be perfect and should never be used as the final say on a players value. In truth, they should be used to spark conversation and discussion and further investigation, not end it. When we see strange results just as much as we shouldn’t assume they are true we shouldn’t assume the whole methodology is worthless.
Furthermore, making any argument against a new methodology because it doesn’t conform to “modern hockey thought” and suggesting they revise it to make it conform more to “modern hockey thought” is plainly the worst thing one can do. The best discoveries in the history of humanity typically arise when people don’t conform to current thought processes but rather do something different. You are free to make an argument against something but make sure that argument is something deeper than “it doesn’t conform to modern hockey thought.”
Finally, my biggest beef with many in the pro corsi/possession/shot differential crowd is the way in which many immediately and abjectly dismiss anything that strays from a corsi/possession/shot differential analysis. This is as fundamentally misguided as those that claim that corsi/possession/shot differential is meaningless and goals are the only tool one should use in player evaluation. The truth is, both methods provide value. The possession method primarily provides value when dealing with small sample sizes as it will reduce small sample size and random variance issues. Shot differential metrics are inherently a flawed metric though because shot differential isn’t the end goal of the player (goal differential is what matters in the win/loss column) and shot quality and ability to drive/suppress shooting percentages exists and are real. There is nothing wrong with using possession metrics as an evaluation tool so long as we are aware of this limitation just as there is nothing wrong with using goal based metrics as an evaluation tool so long as we are aware of its sample size, randomness and uncertainty limitations. Neither are perfect, both have their uses, both have their limitations and in reality both should be considered in any player evaluation.
(Note: Just to be clear, because apparently Tyler Dellow has a poor ability to interpret words properly, my critique of Eric T’s critique of the goal based all-encompassing player evaluation metric does not in any way mean that I believe Dwayne Roloson helps his team score goals. To be completely honest, I serious question how the authors of the paper incorporate goalies into the methodology and this is supported by the fact that in my own all-encompassing player evaluation metrics – goal or shot based – I assume goalies have no influence on a teams offensive production. Hope this clears the issue up for Tyler.)