Linking penalties and game minute in the NHL

By Ingrid Rolland and Michael Lopez

At the 10-minute mark of the first period during Game 2 of Tampa Bay’s 2nd-round series with Boston, Torey Krug was sent to the box for two minutes after committing this slashing violation against Brayden Point.

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The Lightning cashed in on the ensuing power play, with Point scoring the game’s opening goal.

Fast forward to later in this same game, with Tampa Bay clinging to a 3-2 advantage and less than four minutes remaining in regulation. Brad Marchand skated past Anton Stralman for a scoring chance, and the Lightning defender reached around to commit what looked to be a similar violation to the one deemed a penalty on Krug above.

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No penalty on Stralman was called, however, and Tampa Bay held on for a 4-2 win. It was the first of the team’s four consecutive triumphs over the Bruins that earned the Lightning a spot in the Eastern Conference Final.

“We hate to harp on the ref’s, but tonight they deserved to get harped on,” opined NBC’s Jeremy Roenick after the game. “How can you call [Krug’s] penalty early in the game, in such a big playoff game?”

 

 

Roenick went on: “And [Stralman’s] is not a slash? That is the definition of the new slashing rule. How can none of these be called at the most crucial part of the hockey game in the playoffs?”

Although there appears no obvious reason that an NHL referee would deem Krug’s penalty a slash but not Stralman’s, there is one critical difference between the two potential violations — how much time remained on the game clock. Krug was sent to the box halfway through the first period, while Stralman’s play came with only a few minute remaining in the game. In this post, we’ll look at which types of penalty frequencies vary by game minute, compare penalty frequencies in the regular season to postseason play, and suggest whether teams can expect a quicker or slower whistle.

When are penalties called?

Before looking in more detail at different penalty types, we first want to understand how the likelihood of all penalties varies over the course of a game.

Below, we see the average penalty rate for each minute of regulation play, split by period. Each black dot represents the average number of penalties called at a given minute (i.e. the number of total penalties divided by total number of games), while the blue line reflects a smoothed curve to more easily visualize the trend over each 20’ of play. The shaded grey region corresponds to our uncertainty in the per-minute trend. We used penalty data featuring all games from the 2005/06 regular through late February of 2017/18, shared with us by @EvolvingWild.

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One of the first things we observe is the upward trend in each period, whereby the penalty rate starts of fairly low in the first few minutes, before steadily increasing and then stabilizing at 0.15-0.18 penalties called per minute (or one every 6 minutes, approximately). Then, during the final minutes of each period, we see a sharp rise in the penalty rate once again. Altogether, the rates of postseason and regular season penalties are somewhat similar.

The tendency for few penalties to be called during the first minutes of play is similar to what tends to also observed in several other sports, such as football and soccer. In the case of the NHL, the data here suggests a penalty is twice as likely to be called during the final minute of the 1st period as it is during the first minute of the game. As with football and soccer, though, player aggression, which usually picks up as the period goes along, is typically one driver of these overall trends.

Are different penalties called at different times?

Based on these initial observations, we then wanted to understand whether the same trends existed across all penalty types and if those differences in penalty rate by game minute were more likely to be a results of player behavior or refereeing.

To do so, penalties were grouped into two categories based on how much subjectivity was required to call the penalty. For example, there are very clear rules that define what counts as a delay of game penalty, therefore making them much less subjective. Other types, such as holding or hooking calls, tend to require more judgment from officials.

Less Subjective Penalties

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For delay of game and slashing penalties, third period patterns are similar to earlier in the game. Additionally, these penalty types seem to be called at relatively similar rates during the regular season and postseason.

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Roughing calls also appear to be quite consistent from period to period, although the last minute of play in the regular season does not increase as much as in other periods of play. Roughing penalties are also more common in postseason play, with rates about 50% higher than in regular season play.

More Subjective Penalties

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For holding and hooking penalties, the pattern in the 3rd period looks significantly different from the first two periods. While the penalty rate starts low and increases slightly during the first few minutes of play in the third period, we see a large drop of violations late in the game for these calls.

In fact, the highest penalty rate for each of these in the 3rd period is roughly equal to the lowest rate by minute from the 2nd period, suggesting referees may be less likely to call these infractions during the final minutes of the game.

For both holding and hooking penalties, similar trends can are observed in both the regular and postseason.

Limitations and lurking variables.

Our analysis suggests that for infractions that involve less referee discretion, the rate with which each penalty is called is roughly similar within each period. However, for violations that involve more judgment, including holding and hooking penalties, fewer penalties are called in the third period, particularly in the last few minutes of the game.

The obvious explanation for our findings is that what warrants a penalty in the first period of a game may not warrant a penalty in the final minutes. Referees may want to avoid being part of the narrative of how a game was decided, and that’s generally done by not calling a 50-50 play a penalty during the final minutes of a game.

However, a few alternative explanations are also plausible.

First, players may change their style of play, either due to strategic reasons or because they may be tired. However, for this explanation to fully account for the differences we observed above, we would have expected rates of non-judgment calls in the third period to also behave differently from the first two periods, when in fact those infraction rates look nearly the same from one period to the next.

A few other drivers of player behavior are relevant, though. These include the game’s score (future work may want to isolate only games that are tied, for example) or the NHL’s point system. During the regular season, teams may play a certain style in order to increase the chances of reaching overtime, and looking at the difference in penalty rates between early and later on in the season might be another path worth exploring.

 

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