Wednesday, December 14, 2011

Assessing Zone Entry Methods

Over at Broadstreet Eric T. and Geoff Detweiler have been collecting and analyzing zone-entry data for all Flyers games this year. They track each entry into the offensive zone, recording who is on the ice, how many shots and scoring chances there were before the puck left the zone and whether the puck was carried in, dumped in, passed in etc. Here is a post archiving some of their previous work. I think it is an excellent idea and the early data look promising.

In recent articles, which can be found here and here, Eric uses early data to make the argument that carrying the puck into the offensive zone is better than dumping it in and teams, or at least the Flyers they've got data on, should be more aggressive, carrying it in more often. He does put in a couple caveats that the fourth line should be more inclined to dump the puck in and top 3 lines should perhaps be more cautious late with the lead. While intuitively I think he's correct, the extra pressure you put on your opponents with the puck seems more valuable than the risk of a bad turnover, I don't think the results he cites tell us anything about whether or not teams should try to carry it in more often in marginal situations. I have a different interpretation of the data.

Their results: teams do better when they carry the puck in.

One thing there is no doubt about whatsoever is that in their dataset teams get substantially better outcomes when they carry the puck in than when they dump it or even pass it in. (Before you ask, they exclude situations where a team dumps the puck and makes a line change with little to no effort to go after the puck) As an example, when the puck was carried in the team doing so generated 0.57 shots before the puck was sent out of the zone. The similar number is only 0.22 when the team dumps it in.

On the face of it, it seems reasonable to think that this means carrying the puck in is smarter and that teams should be doing it more often. However, this ignores the circumstances. Most of the time when a player can easily carry the puck across the blue line into the zone it is both correct to do so and what he does. These situations tend to overwhelmingly favor the attacking team. In extreme cases you have breakaways and odd-man rushes. In general the defense will not be very well set up - if they were the offensive team would not be able to waltz into the attacking zone without risking losing the puck.

Now think about times where it would be very difficult for the player to cross the blue line with possession of the puck. The defense is set up, putting pressure on the puck handler. He is likely to be facing a very good defenseman. He might even have multiple defenders perfectly executing a trap. In these situations, dumping the puck is the correct move and usually what is done.

Looking at the two together, when the situation is favorable to the team about to enter the offensive zone they tend to carry it in. When it is unfavorable, they will usually dump it. Let's flip that around - when teams carry the puck in the conditions are usually very good for the offensive team and when they dump it in they are usually bad. I think it's pretty clear that the circumstances would drive the numbers in exactly the way they appear. From the numbers alone it's not clear whether or not the teams in question attempt to carry the puck into the offensive zone too rarely, too often or about the right frequency. Intuitively I agree with Eric's conclusion, but I don't think the data provide any evidence for it.

Let's shift the focus.

In my view Eric focused too much on the decision the puck carrier makes and not enough on what is happening on the ice when, and just before, he makes it. I'm not saying this to be negative, in fact it's quite the opposite. He, Geoff and Broadstreet in general write some of my favorite hockey stuff, and that's saying something since I'm a Pens fan and the Flyers are my least favorite team. I love the idea of looking at zone entries and, perhaps paradoxically, my interpretation puts more value in these metrics than his does.

Let's take a different view of some of the data from the three recent articles, including comments:
- the top line (Hartnell, Giroux and Jagr) carried the puck in 3.1 times as often as they dumped it in. For second and third liners this drops to 2.2 times as often and for the fourth liners it's all the way down to 1.4 times. Better players tend to carry the puck in more compared to dumping it.
- when the puck is carried in the results are better than for any other type of entry going by shots/entry, chances/entry, goals/entry and how often the next play is in the defensive zone.
- the team carrying the puck in gets the next shot off 69.8% of the time, just above passing it in (68.6%) and well above both deflecting (62.4%) and dumping it in (56.3%)
- carrying the puck into the zone is substantially more advantageous than getting a faceoff in the offensive zone, going by any of the above metrics.
- when the Flyers have a lead of 2+ in the first two periods or any lead in the third, so their opponents are taking risks, 58% of their zone entries are carried in or passed in where they maintained control. When trailing, their opponents are more defensive minded, this figure drops to 48%. When the game is close and the opponents are more balanced it is in the between at 55%.

What does all this say to you? To me it screams out that the ability to carry the puck, or pass it in with control, is a fantastic proxy for winning the neutral-zone and transition-game battle! Giroux isn't good because he carries the puck in, he carries the puck in because his strong play has given him the opportunity. The more the conditions dictate the decision, the better controlled entries measure how well teams and players are doing in the neutral zone. It is precisely because as I see it the conditions drive the entry decisions that I think it's such a good thing to track.

Conclusion and Suggestions

While Geoff and Eric only have 22 games so far this season, a preliminary look at the data indicates that zone entries, especially those where the puck is carried in, may tell us a lot about who is excelling in the neutral zone, on transitions from offense to defense and vice versa. This is quite promising and I have a few quick suggestions for ways they could use these stats.

Firstly, I would put it in percentages as we do for Corsi. In other words carried-in entries for divided by carried-in entries for and against. I would also consider weighting them differently to come up with one overall number. People often ask about high-value shots and other notions related to shot quality. In this case, there appear to be high-value entries and low-value entries. Another thing to look at is how often the opponents carry it in with different defenseman on the ice.


  1. As we talked about on BSH, I agree that when I talk about marginal strategy changes I veer from "proven by data" to "my hunch based on the data".

    And like I said there, my hunch is partly based on the preconceived notion that in sports, people generally tend not to use boom/bust strategies as much as they should because the downside stands out too strongly in their memories -- teams play for one run too much in baseball, players don't go for enough on their second serve in tennis, teams don't go for it on fourth down enough, etc.

    Dumping it in achieves so little that my intuition strongly suggests that teams will be better off either forcing a controlled entry or backing up and trying again when carrying it in does not appear to be easy. And sports history tells me that fans and coaches probably hold failed carry-ins against players more strongly than they should.

    As for the idea of putting it in percentages...that's something we have to do with Corsi to remove scorer bias, but it costs us information about whether a player's success is at the offensive or defensive end. Since the zone entry project only has a single scorer so far, I don't see the advantage of collapsing offense and defense into a single net score.

    I have looked at how often the opponents carry it in with different defensemen (see the summary after 7 games, for example). I left it out this time around because frankly it didn't make for an interesting story -- the results were mixed and I wasn't sure what to make of it. I think the problem is that the best defenders also tend to face the best forwards, who are in turn the most likely to carry the puck in. With a league's worth of data, you could sort through that by comparing the carry-in percentage to what the opponents average (weighted by ice time against), but with one team's data I didn't see how to get there.

  2. We're somewhat going in circles here, and again I think we are 90% in agreement.

    Like I said in the article and over at BSH, I agree with your intuition and suspect teams probably do dump it in too often. I've wondered why teams don't back up and try again more often and even why they don't use more extreme strategies like passing it back to the goalie for line changes instead of just giving up the puck.

    You definitely make a good point on the scorer. A reason I like percentages are standardizing it so it looks like what we're used to with Corsi/Fenwick. Maybe a better suggestion is to use something equivalent to Corsi rate where it's controlled entries for minus controlled entries against divided by TOI/60. I think there is an advantage to combining them, which is that it tells you how the neutral-zone/transition battle is going overall.

    Breaking it down in more detail and looking at the percent of the time guys carry it in as you've done is great but I can think of no better way to get a quick grasp of who is winning the battle in the middle than either a +/- rate or Corsi-style percentage with controlled entries. I think you've really got something there.

    As for defensemen and carries in against, I see your point. Maybe once the season gets late you can do some small-sample pilot type stuff by looking at opponents you play several times in something like WOWY but with opponents. Look at how each defenseman or pairing does against the Ovechkin line, for example. Or maybe you could group them somehow by TOI or Corsi or something to see how they do against top players. Either way, you'll have to get more data and even then be dealing with some small sample sizes.

    Anyway, just some suggestions, I look forward to seeing what else you come up with.