How an NHL rule change affected player behavior
As a behavioral scientist and pro hockey fan I often wonder how changes in the NHL affect player behavior. These could be player or line changes, goalie changes during games, coach changes mid-season, or rule changes. In the 2017-2018 season I had my opportunity to measure player behavior based on a rule change. Among the rules changing for that season the NHL decided to be more strict on what they would consider slashing. Here is what the NHL said regarding the rule change:
Rule 61 – Slashing:
To curb slashing in and around the hands, there will be a stricter standard of enforcement. Referees have been instructed that when a player is NOT attempting to play the puck and slashes on or near an opponent’s hands, such player should be assessed a slashing penalty.
Many people on the internet think the particular incident below was the impetus for the rule change. WARNING: GRAPHIC CONTENT
As the 2017/2018 season began, I noticed there were a lot of slashing calls being made. I wasn’t the only one noticing either - the announcers for the Blackhawks also commented on the rule change, how it was being implemented, and the differences between what would have been a no-call in seasons prior vs penalty calls post rule change. I only really watch Blackhawk's games during the regular season but there definitely seemed to be an increase in slashing calls.
The Question(s):
What was the result of stricter enforcement on player behavior?
Did penalties increase, decrease, or stay the same?
How did these data compare to years past and future?
If there was an increase, how long did it take for the data to plateau, if it did at all?
Did the stricter slashing rule result in a decrease in hand/wrist injuries?
The Method: I used Icydata.com to look up all the slashing penalties for the 16/17, 17/18. and 18/19 seasons. I transferred this data into excel and used pivot tables to map the slashing calls by date and compare to the years before and after.
I also used nhlinjuryviz to look at the types and frequency of NHL player injuries to see if the rule change had an effect from years prior.
The Results:
Below are the slashing penalties over time for the 2016/2017, 2017/2018, and 2018/2019 seasons:
A visual analysis of these data show that the beginning of the 2017/2018 season, when the stricter enforcement was active, was rife with slashing calls. The highest number of calls made was 37 on 10/8/17. The number of calls regress towards the mean as the season continues. From 10/26/17 onward the data mimic the 2016/2017 season and the 2018/2019 season. The 2016/2017 and 2018/2019 season do not show the large spike at the beginning of the season.
Another way to look at this data would be slashing penalties per game (thanks to Icydata.com for the viz):
The sharp increase in slashing penalties per game is obvious here. The premise here is that if the rule enforcement became more strict you would see a decrease in slashing penalties as the players adjusted their behavior to the rules.
Injuries
Since the underlying premise of the rule change was to protect players upper arms, hands, wrists and fingers, what does the data show in terms of frequency and types of injuries?
High fidelity injury analyses are difficult because the NHL are notoriously secretive about player injuries. What I have tried to do is break down major injury stats so that comparison across seasons could be made. First we will look at total injuries and upper body injuries per season:
This chart shows a small dip in total injuries and upper body injuries from the 16/17 season to the 17/18 season. Not too much can be concluded from this graph based on the question that was asked above. Let's go a step deeper:
There are a few specific upper body injuries that would be likely injuries for slashes to the upper arm. The 17/18 season saw an increase in Hand injuries, but a decrease in finger and wrist injuries. It would be difficult to say, for certain, that the rule change caused the decrease in wrist and finger injuries as there might have been other factors causing these incidents, and there is also probably a range of acceptable injuries when one plays a physically demanding sport.
Discussion
The slashing rule change definitely increased the number of slashing calls made during the season, and there was a large uptick in slashing calls during the first month of game play. The results of the rule change seems to have reduced some of the arm/hand injuries in the process, but it would be difficult to tie the two causally, as there are many other factors in play (most important of all the secretive nature of NHL injuries).
Comments