We credit two assists on most goals. But do both passers really deserve credit?
The numbers suggest that secondary assists have very little connection to a forward's talent level, and that we would do better when evaluating forwards to ignore them altogether. After the jump, we'll walk through the data that supports that conclusion and talk about its implications.
Certainly there are some instances where the secondary assist is a valuable contribution. Here's an example where JVR's pass creates the scoring opportunity and he completely deserves the point that he's awarded:
And yet there are also instances where the secondary assist is not much of a contribution. Does anyone think that the point that Ville Leino registered in the video below from the same game reflects any particular skill on his part?
Of course, we could pick out individual examples all day and not get any closer to answering the general question. We need to frame a question that looks at whether secondary assists reflect a talent in general.
Here's the approach I'm going to take: I'm going to assume that if something does reflect a talent, then it should persist from year to year -- you should expect that how a player does in one year will be at least slightly predictive of how he will do in the next year. If that's true, then we'd expect to see a correlation between players' results in one year and their results the next year. (Here's a reminder of how we look for a correlation.)
I expect to see quite a bit of fluctuation -- from shooting percentage luck, from teammate changes, from injuries, from aging, etc. So let's start by removing whatever sources of variation we can.
- We will use only even strength numbers, so we won't get fluctuation from people's power play time (or luck) changing.
- We'll use rate statistics (events per 60 minutes of ice time), so that we won't get fluctuation from people's ice time changing.
- We'll only compare seasons where the player played at least 60 games in both seasons, so that we reduce the fluctuations that come from luck in short seasons. This still leaves us with 460 data points from '07-08 to '09-10, which should be plenty for drawing out trends.
All of that will help, but there will still be quite a bit of fluctuation. So let's start with something we all know to get familiar with these numbers and get a sense for what we should consider significant. We pay a lot of attention to goal scoring, so we have an intuitive sense for how much goal scoring changes from year to year. Let's look at what the numbers say about it.
So now you're calibrated -- something with a correlation coefficient (R2) of 0.22 fluctuates about as much as goal scoring does. A correlation of 0.22 actually means quite a bit of fluctuation (just ask Alex Ovechkin), but I think we all believe that scoring goals is a talent, so this tells us that we should consider a correlation near or above 0.22 to show that something is a result of a player's talent.
Next, let's see how much primary assist rate changes from year to year.
The correlation coefficient of 0.18 is a bit lower than for goal scoring, but there's still a clear persistence of primary assist rate. Someone who sets up a lot of goals this year probably has vision and puck control skills that make him likely to do so again next year. Is the same true of secondary assists?
No. The correlation rate on secondary assists is much lower, only 0.05. If talents like vision and puck control help a guy get secondary assists, they sure don't help much -- the guy who leads the league in even strength secondary assist rate one year is only slightly more likely than Jody Shelley to lead the league next year, and he's almost as likely as not to be in the bottom half of the league.
Let's make one more refinement on this analysis. Since you need a teammate to score the goal, we might expect that assist numbers would show a significant dependence on your teammates. What happens for the guys who change teams?
Let's start by looking at primary assists. Overall, the correlation between what players do one year and the next was 0.18. Is it much lower when there's a team change in between years?
No. When a player moves to a new team, he takes with him the playmaking skills that allowed him to have a high (or low) primary assist rate -- the correlation is nearly as high as if he didn't change teams. Let's ask the same question about secondary assists.
We saw very little persistence of secondary assists in general (0.05 overall, see above). And now we can see that whatever persistence there was appears to be entirely due to the quality of the offense the player played for -- when he changes teams, there is zero year-over-year correlation in his secondary assist rate.
So in the end, we're left to conclude that -- at least for forwards, at even strength -- secondary assists aren't really a talent at all. By including them in people's point total, we're just adding noise, and making it harder to tell who's having a good season. Here are a couple of examples where that may have steered us wrong.
- Last year, Claude Giroux averaged a pathetic 0.06 ES A2/60 (even strength secondary assists per 60 minutes of ice time) despite a passing skill that allowed him to have 0.95 ES A1/60. The low secondary assist rate probably cost him about 10 points and kept the world from getting as excited about him as we were.
- Derek Stepan might be similarly underrated for the Rangers this year. His A1/60 rate is a very nice 0.74, but his A2/60 rate is 0.20. A little more luck on the secondary assists and he'd be a strong Calder contender.
- Jamie Benn looks to be a very nice player, but his year-over-year growth isn't quite what it looks like. He's benefitting from the 10th highest secondary assist rate of any forward this year, and we shouldn't count on that continuing.
Here's the good news: because many goals don't have a secondary assist, secondary assists make the smallest contribution to point totals. So even though secondary assists just add noise (luck), they don't add enough noise to do too much harm. But since they certainly don't seem to add any value, let's focus on goals and primary assists when evaluating forwards from here on out.