Thursday, March 20, 2014

An introduction to on-ice percentages

One of the things I like to do in this space, in between reminiscing about the Norris Division and breaking down hockey player lip sync videos frame by frame, is occasionally dip a toe into the advanced stats waters. So far this season, we’ve already introduced most of the basic concepts, walked through how they can be used to project success or failure for a given team, and looked at the surprising importance of zone entries.

Today, let’s take a crack at on-ice percentages. It’s a concept that presents a slight variation on some common stats you’re already used to, and it doesn’t involve any especially onerous math, but it can end up being crucially important to understanding a player’s more traditional stats. And that turns out to be especially true when those stats start telling us something we weren’t expecting.

Let’s start with on-ice shooting percentage.

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What is on-ice shooting percentage?

You’re already familiar with a player’s regular shooting percentage, which is a basic stat that most fans understand well. Shooting percentage is simply the percentage of shots on net taken by a player that result in a goal. A player who takes 300 shots and scores 30 goals is shooting 10 percent. That would be better than the league average, which over the last several seasons is right about 9 percent.

On-ice shooting percentage is essentially the same concept, but with an important twist: It counts all shots taken at the opponent’s net when a player is on the ice, including those by his teammates. It’s a measure of how successful a team is at converting its shots with a certain player on the ice, whether or not the player is the one doing the shooting. Like any stat, on-ice shooting percentage can be applied to all game situations or broken down further by specifying even strength, close situations, etc.

It’s a fairly new stat, since it relies on data we’ve only had available to us for a few years. Unlike regular shooting percentage, on-ice shooting percentage isn’t typically mentioned on TV broadcasts or in newspaper stat packages. But it’s become relatively easy to find online, and you can now pull on-ice shooting percentages for every player on sites like ExtraSkater.com (where it appears on every player page as “Sh%” under the dashboard’s “5 on 5 on-ice” header).

Why should we care?

It should go without saying that on-ice shooting percentage will have an enormous impact on a player’s more traditional stats. If a high rate of his own shots go in, he’ll score more goals. If a high rate of those taken by his teammates go in, he should expect more assists.

And unsurprisingly, that’s exactly what we see. Many of this year’s top scorers, including guys like Evgeni Malkin, Ryan Getzlaf, Sidney Crosby, and John Tavares, also rank in the top 20 for overall on-ice shooting percentage among players with at least 52 games played.

That’s nice, but not necessarily all that useful — after all, we don’t need advanced stats to tell us those players are having good years. The real value here, like with most advanced stats, would lie in helping us make educated guesses about the future, and which players are most likely to sustain their level of performance.

On-ice shooting percentage could do that, but first we need to know whether it’s primarily driven by skill or luck. If it’s a skill, we’d expect players with high percentages to keep racking up points. If it’s mostly luck, we’d expect them to be top candidates to fall back to earth.

So which is it? Let’s find out.

>> Read the full post on Grantland




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