In an attempt to understand and de-mystify the advances statistics involved in hockey, I’ll be posting a few blogs to break them down. Firstly, hockey calls them advanced statistics even though they’re not particularly advanced, and secondly the NHL is very slow on the implementation of stats in comparison to the MLB or NBA, thirdly statistics are nothing to be freaked out over.
One of the easiest stats to figure out is plus/minus (+/-). A player gets a + every time he is on the ice for an even strength goal. A player gets a – every time he is on the ice for a goal scored against the team. Fairly easy, right?
The table below shows the top five and worst five +/- scores from the 2015/2016 season. I wouldn’t have predicted those guys. Can you imagine a top 5 without at least Crosby, Ovechkin, or Kane popping up? The problem with +/- is that it’s great in theory, but in practice it is too basic. Good players with bad team mates get dragged to hell, whereas bad players with good lineys get their score boosted.
|Tyler Toffoli||+35||Mikkel Boedker||-33|
|Anze Kopitar||+34||Radim Vrbata||-30|
|Brian Campbell||+31||Bo Horvat||-30|
|Chris Kunitz||+29||Jordin Tootoo||-26|
|Colton Parayko||+28||Elias Lindholm||-23|
It’s a good way to distinguish good defensive players, but it fails to explore the quality of team mates, the competition, goaltending, or how a player is used in the game. And this is important contextual information. You should never look at the statistics in isolation, it needs context.
Let’s take a look at our favourite Russian, Alexander Ovechkin. Between the 2013-2014, Ovi posts a -35. Holy cow. That’s third worst in the entire league, only beaten by Steve Ott and Alex Edler. But if we look at the goals he scored that season, it’s the highest in the league with 53. If he scored that many goals, why is his plus/minus score so atrocious? He’s a lazy player. He’s not a good defensive player. He hogs the puck. Okay, we know he’s not the best defensive player, but there has to be more to it than that. As for assists, he recorded 28 that year – which is by no means bad – but actually he was making the passes, but his teammates were not scoring from them, (we’ll cover that later), and the goaltending was shaky. But if we know nothing about hockey and look at this easy to understand stat then we would most likely assume that this Alexander Ovechkin guy is an awful player.
To further illustrate how the plus/minus figure can be easily skewed, here are some examples.
Scenario: We have two identical hockey players (who happen to be the Sedin twins) but let’s say they shoot ten times in sixty minutes and allow ten shots in sixty minutes on ice. They both have an on-ice shooting percentage of 10% and an on ice save percentage of 91%. Both players play 20 minutes per game. Every play, skate, pass etc is identical.
Example 1: Player A plays 20 minutes every night, but now Player B plays only 10 minutes. Over the season Player A will have double the plus/minus score as Player B because he is playing for longer each night to hit that 10 goals per sixty minutes of ice time. It ends up looking like Player A is twice the player that B is when they are actually identical players.
Example 2: Both Players are identical again, and both play for the exact same amount of time. However, the goalie for Player A’s team records a save percentage of .930, and Player B’s goalie has a save percentage of .890. More goals will be scored against Player B’s team so he will get a bigger minus and so Player A has the better +/- score – despite goal tending having nothing to do with them.
The take home message is that a plus/minus score is highly variable and many things can sway it. You could get a +3 from Monday’s game and a -5 from Tuesday’s. It’s not the best method to use and ignores context! So please can we abandon it for good?