How Much do Defensemen Drive Play Compared to Forwards?

Look at the most expensive players in the NHL and you will find that the majority are forwards. There is definitely a disparity in how teams pay free agents based on position. Do not mistake this point for the idea that defensemen do not get paid; they do. Filter the list of players from the link above to just defensemen and we still have a list of highly payed professional hockey players. Now, take a look at the highest paid forwards. If you do not feel the need to click all of those links, take my word on the following; if you do you can come up with your own conclusions: subjectively, the list of the highest paid defensemen is riddled with a higher proportion of albatrosses. In the hockey fan community, there is definitely a consensus that the league is worse at evaluating defenseman. The box-score statistics do not do an adequate job of describing or predicting a defenseman's value, especially relative to forwards (where points, which are still not a great indicator of talent, have more standalone value). Part of the issue is that aforementioned discrepancy between gauging the value of forwards and defensemen. But another part of the issue is, based on the roles each position plays on the ice, defensemen just matter less and their on-ice results are subject to way more noise than forwards.

I am going to define offensive and defensive play driving as the differential between a team's shot attempt and expected goal rates (xG for the rest of the post) when a given player is on or off the ice. These are called relative metrics and are displayed on any of the advanced stats sites available for public consumption (for this post all my data is from Natural Stat Trick). The obvious drawbacks for using these measures to approximate play driving ability is they lack context. They do not account for who the player plays with or against (teammates are more important), zone starts, score state, home versus away, etc. But for the purpose of this study, I think they will do. If you believe that the results are invalidated because of the way I chose to use the data, I cannot stop you from doing so.

The data set includes skaters from the past two regular seasons who played at least 200 minutes at 5v5 in both seasons (all the analysis is done looking at performance at 5v5). I first looked at the changes in each measure of offensive and defensive play driving and how they changed for each player year over year, by position.

In each instance, forward contributions are less variable year over year, even with regards to defensive play driving ability, though defensive performance is more variable overall. We should be less confident in how defensemen drive play in a given season compared to their forward counterparts. Theoretically this uncertainty should manifest itself in how we understand a given player's mean and distribution of performances in a season or set of games (defensemen have more variance, thus we should be less confident). 

I built four simple linear models for each of each of the measures of offensive and defensive play driving with the goal of predicting 2019-2020 results from the 2018-2019 results and a player's position. The players position (either forward or defenseman) was statistically significant in each case. Based on the collection of plots above, offensive play driving had much less noise than defensive play driving.

Offensive shot attempt driving had the least amount of variation, followed by driving xG on the offensive end. Therefore, we should be much more weary of attributing defensive results to defensive ability and instead chalk a lot of those results up to variance (which should still be done on the offensive end, but as I alluded to much less so). With that being said, looking at the distributions of play driving results on offensive paints a grim picture for defensemen.

Forwards have higher positive and negative impacts and less of the population is concentrated around the mean. This is another indicator that forwards matter more; they have a larger effect on the most predictive parts of a player's value.

What can we draw from this? First, whenever a team signs a defensemen to a massive long-term contract, our ears should perk up. I am not saying that no defenseman deserves a 40, 50, 60, 70, or 80 million dollar contract. I just want to caution fans on how confident they should be when considering the merits of a defenseman, especially compared to forwards. Considering that defenseman play driving ability is more variable year over year, I think teams should really be investing in their forward corps more and taking fliers on cheaper defenseman on short term deals, where they can either be useful pieces in the lineup, good trade chips, or sunk costs that are not too expensive (as Kyle Dubas has done in Toronto, to the dismay of many despite the team's consistent success the past few years). Keep this in mind during the next trade and free agency frenzy, sometime in October.