Friday, July 8, 2011

On Power Play Points By Defensemen And The Limits of Knowledge

Is there any more of a buzzkill blogpost title than 'The Limits of Knowledge'? No, probably not. Still, I like exploring questions more than I do necessarily getting the answers.

One of the things that bothers me about hockey in general is that single unit of scoring, the point. Do we know why secondary assists are given on the vast majority of goals? Why is that the standard? How often have you watched a game and seen assists given for a blocked shot or goalie's save? Yet, these points are the way in which most hockey fans judge a player. If he's got a lot of points, he's good, if he doesn't, he's not. While this is largely true for forwards, for defensemen it creates a problem, because there is no true 'defense' stat. There are hits and blocked shots, but measurements of these things vary wildly from arena to arena. Plus/minus measures offense relative to defense, so that's really no help. What I see fans doing too often is relying on points, even though I think most of a defenseman's job is to prevent goals, not to contribute to the scoring of them.

So, as is my custom, I made a spreadsheet. I wanted to examine power play points, as it is my belief that not all points are created equal. Why should we necessarily reward defensemen who get a lot of points on the power play? Are they better than their peers at the power play - so much so that they should be heavily compensated, as James Wisniewski was this off-season? So this spreadsheet lists every defenseman who played 40 or more games last season and got at least 2 minutes per game on the power play, the number of points they got, the number of total PP minutes they got, their points per 60 minutes of PP ice time, and the shooting percentage* while they were on the ice *note: shooting percentage only covers 5 on 4 PP time. Voila. (Stats courtesy of and the incomparable

That looks all well and good, but what does it all mean? As is all the rage these days, I ran a Pearson's correlation on some of the important numbers. A Pearson correlation takes two sets of variables and shows how closely related they are. A Pearson correlation ranges from -1 to 1 - -1 means they are inversely correlated, 1 means totally correlated.

First, I looked at PP Time and Total PP Points, which is silly - they are of course highly correlated, and I got a Pearson value of .84, which duh. Next I looked at the relationship between Points/60 and Power Play S%, and I got a high value there - .66. Next I looked at the relation between total power play time and Points/60, and I got a reasonably high value - .55. Lastly, I looked at the correlation between power play points and shooting percentage. All of these, according to the good people at Wikipedia, are strong correlations. Let's list them out:

PP Time/Total PP Points: .84
Points/60 - Power Play S%: .66
Total PP Time - Points/60: .55
Total PP Points - Power play S%: .50

Okay, so to put this in simpler terms, I was asking, aren't guys who get a lot of points on the power play also just guys who get a lot of power play time? This suggests, yes, that is pretty much the case. Okay, and aren't a lot of power play points related to power play shooting percentage, and this says, yeah, a lot of the time, that is true. We've generally seen that shooting percentage at even strength is more luck than skill, so why shouldn't that hold true on the power play? With all that being true, I looked at power play time and points/60, and basically the more time on the power play a player gets, the more points/per 60 minutes he scores. And back again to the beginning, the points a D man gets on the power play are related somewhat closely to the shooting percentage while he is on the ice.

So didn't I prove what I set out to prove? Not really. We've once again run into the biggest problem in hockey: Usage. It's especially true on that third correlation - total power play time is correlated with a player's points/60. What this means is that the more points a player gets, the more time he will end up getting. All this time we've been looking at individuals, but the problem with hockey is that A: it's a team game and B: the coach chooses at almost all points of the game what players he wants out on the ice. So the fact that the players better at generating more points get more ice time isn't at all an accident - their coaches are putting them out there more often because ostensibly they're better. And the fact that the power play shooting percentage is higher with those players on the ice might not be an accident either; the first unit power play shooters may be more skilled. We also have the problem of taking such a limited sample - 300 minutes of ice time is equal to around 20 games' worth of even strength ice time for these defensemen. We'd need multiple years of data to try to prove anything substantial.

The overall relationship between defensemen and offense is nebulous, and this study hasn't really helped to clear anything up. Still, unlike some sites that will just publish anything even with faulty logic at its heart, Driving Play will admit when its studies don't reach a logical conclusion. I may follow up on this next season.

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