Possession, Shooting Percentage and Outlier Teams
Shot quality as a talent at the team or player on-ice level has long been a topic of great debate and I outlined some of that debate in an article I wrote earlier in the week. For those who don’t believe that shot quality is a significant factor in performance put a lot of stock in possession metrics such as Fenwick or Corsi. These are shot attempt based metrics and as such ignore shot quality altogether. For those, like myself, who believe shot quality matters (at least for some teams and especially some players) I consider a possession based analysis a (potentially) incomplete analysis. Today I am going to put that debate aside and ask the question, is there any relationship between possession and shooting percentage?
To answer this question I took a look at CF% and CSh% (Corsi shooting percentage = GF/CF) for all 30 teams over the previous 3 seasons combined in 5v5close situations. When I plot these, here is what I get.
Ok, so while there seems to be some correlation it really isn’t all that significant. You might be inclined to end the investigation right here and conclude that there is no relationship but when you actually look at the data you will find that of the 10 best CSh% teams 8 of them are sub-50 CF% teams and of the 10 worst CSh% teams six are better than 50 CF% teams. The two top CSh% teams that have CF% above 50% are Chicago and Pittsburgh, two teams with elite level talent. The four bad CSh% teams that have a CF% below 50 are Florida, Carolina, Minnesota and Buffalo. Of those teams, Florida, Carolina and Buffalo have combined for one playoff appearance in each of the past 3 seasons.
So, it appears that the teams that break the trend of good CSh% equals poor CF% and poor CSh% equals good CF% are the truly good or truly bad teams or, for better terminology, we could call them outlier teams. What if we attempted to remove the really good and really bad outlier teams from our analysis and focus on the teams that are more typical teams in terms of talent. To do this in an unbiased way I used GF% to rank teams and I removed the top 4 and bottom 4 GF% teams (8 total, or just over a quarter of the teams were removed). This is what the chart looks like now.
Now that looks better. R^2 has jumped from 0.09 to 0.47 and there is a clear negative relationship between possession and corsi shooting percentage. For the record the teams that were removed were Boston, Anaheim, Chicago, Pittsburgh, Calgary, Buffalo, Edmonton, and Florida.
For curiousity I took this one step further and removed the next two best (St. Louis, Detroit) and two worst (NY Islanders, Minnesota) teams and got the following chart.
Wow, R^2 jumps all the way to 0.77 which is a very strong correlation and indicates that for a large number of non-elite, non-terrible teams there is a strong negative correlation between possession and shooting percentage such that the difference between a 45% and a 55% possession team is 1.22% hit to CSh%. Considering last season the average team had about 2200 5v5close Corsi For events that would equate to a difference of about 27 goals. Considering the average NHL team had 90 5v5close goals last season, that is not an insignificant number.
How does the R^2 hold up for this season? Well, if we include all teams the R^2 is 0.00 or absolutely no correlation. If we delete the top 4 and bottom 4 GF% teams it improves to 0.097. If we drop the top 6 and bottom 6 it jumps to 0.26 and if we drop the top 7 and bottom 7 teams and just focus on the middle 16 the R^2 jumps up to 0.35. Now these correlations are not near as good as the 3-year analysis above but remember that our sample sizes are significantly smaller too (~43-45 games compared to 212 games). The general trend still continues. If we remove the really good and really bad outlier teams there appears to be a relatively strong negative relationship between possession and shooting percentage.
Now that we have identified a relationship, on thing we can do is look at how teams have changed from last season to this season. Let’s take the Edmonton Oilers as an example since they have improved their 5v5close CF% quite significantly this season but they are not an improved team. Let’s look at their numbers from last season and this season.
So, their 5v5 CSh% has improved from 43.4% to 48.7%. If we plug that 5.3% improvement into the regression equation above we would expect that their CSh% would drop 0.65% where it actually dropped 0.89%. Edmonton dropped from 11th in CSh% last season to 27th this season.
A couple of months ago I investigated the relationship between Corsi Against rates and save percentage and found that there does appear to be a relationship such that an increase in corsi against would result in a improved save percentage. This is completely consistent with the analysis above which one could infer that an increase in shot attempts correlates with a decrease in shooting percentage.
It is difficult to say whether these correlations are due to systems or talent but I have a couple theories.
- Good possession teams play in the offensive zone more frequently and the defensive zone less frequently. This could result in a shot type bias away from higher quality “rush shots” and towards lower quality zone play shots.
- It could be related to style of play and passing. It has been shown that shots after passes are more likely to result in goals and lateral movement, especially passes, across the “Royal Road” down the center of the ice also result in more goals. My theory is passing, and in particular passing through the center of the ice, while more likely to result in a goal is also more likely to result in a turnover. Thus teams that take riskier, longer passes especially lateral passes are more likely to see plays result in a goal if successful or a turnover (and no shot from that possession) if unsuccessful. Conversely a more conservative passing team with fewer cross-ice passes through traffic would have fewer possession not result in shots but in turn not get rewarded with high quality shots that result from those risky cross-ice plays.
In conclusion, if you have exceptional talent such as Pittsburgh with Crosby and Malkin or Chicago with Kane and Toews or exceptional depth like Boston or Detroit you might be able to be a good possession and a good shooting percentage team but if you are not one of the truly elite teams in the league it seems you likely have to choose one or the other. Unless of course you are Buffalo and you are terrible at both.
Update: Tyler Dellow, in one of his few hockey related tweets since being hired from the Oilers, tweeted the following:
— Hon. mc79hockey (@mc79hockey) January 17, 2015
Tyler is right. Things fall apart for earlier seasons. Let’s look at this in more detail by looking at R^2 between CF% and CSh% for individual seasons for all teams, middle 26, 22, and 18 GF% teams. Here is what we have:
All of the above relationships are negative relationships meaning improved CF% led to decreased CSh% so it is very difficult to argue that this relationship isn’t real. More shots tends to mean lower shot quality.
Additionally, for 5 of the 7 seasons the middle 22 are better than the middle 26 which is better than all 30 teams (only 2007-08 and 2010-11 do not fit) and of those 5 seasons, four of them also have the middle 18 teams being better than the middle 22 (only 2011-12 is worse). This implies that there may be a few truly elite teams that can post a good CF% and a good CSh% and a few truly terrible teams that put up bad CF% and bad CSh% but for the mass of teams in the middle the trend holds.
Finally, the strongest relationships have occurred during the previous few seasons after removing the outlier teams from the sample and from above 2014-15 appears to following that trend as well. It is difficult to say why this is but it is an interesting observation. One has to wonder if it has anything to do with teams becoming more aware of and putting more focus on possession which in turn is strengthening the negative correlation with shooting percentage.