#BlameBuff: The uncontrolability of on-ice save percentage

USA TODAY Sports

Dustin Byfuglien has always been a target. Now he is a trade target as well. Would Jets get full value for Big Buff?

Earlier this month we discussed how a defenseman's largest contribution in improving a team's chance at winning is in how they affect their team's (and opponent's) shot metrics. What wasn't discussed as in-depth was the reasoning why, other than a passing mention that defensemen have little sustainable control over their goaltenders save percentage.

Usually when I discuss general ideas and concepts, I like to look for specific examples and applications within the Winnipeg Jets organization. Byfuglien is usually the go-to case study for shot quality versus shot quantity arguments.

Below is the the shot quantity effect of Byfuglien. The blue line represents the percentage of shot attempts a team has seen with Byfuglien (as a defensemen only) on the ice, while the red line is the percentage of shot attempts the same team without Byfuglien on the ice.

Buff_medium

As you can see, there is a clear difference in the team's shot attempt differentials, despite Byfuglien tending to play against the other team's top lines more than the other defensive pairs. A large portion of Byfuglien moving towards the line has been the ever increasing separation of the Jets two best defenders: Dustin Byfuglien and Tobias Enstrom.

In theory, if you out shoot your opponent you should outscore more often than not, but this is not always the case.

There are two large factors that affect the jump from shot differentials to goal differentials; they are shooting and save percentages, which when combined is known as PDO.


Sh% Sv% PDO
2011 7.90% 0.9198 99.88
2012 7.91% 0.9060 98.51
2013 8.39% 0.9112 99.51
2014 7.13% 0.8889 96.02

The average first pair defenseman tends to see a on-ice shooting percentage of 8.42% and an on-ice save percentage of 0.916 with a PDO just over 100.

In all four years, Byfuglien's PDO is below 100. A sub-100 PDO also means that a player's goal differential will be lower than their shot differential. The low PDO seems to be caused by a mix of both factors, although more save percentage than shooting percentage. Some would say this is due to Byfuglien's poor defensive play, but numbers folk would disagree... why is that?

Eric Tulsky has previously shown that there is very, very low repeatability in on-ice percentages to the point it could be entirely by chance. However, he does also note that the low repeatability may be understating how uncontrollable the percentages are for defenders by ignoring that certain players will play predominately in front of a particular goaltender and under certain systems.

One way to account for this is by looking how a defender's on-ice percentages values are relative to the team's without the player on the ice.


ΔSh% ΔSv% ΔPDO
2011 +1.64% +0.002 +1.79
2012 +0.34% -0.019 -1.52
2013 +0.13% -0.002 -0.11
2014 -1.06% -0.041 -5.19

All of a sudden there seems to be a bit less of a pattern. What was once prior a consistently poor percentages appears to be bouncing around.

Now some will still try to converge numbers into a pattern and make reason out of it. It's human nature. So let's see what analytics say about these numbers for all of the Jets defenders.

Here is the repeatability of single season on-ice relative shooting percentages for all Jets defensemen to play more than one season. The x-axis is any season a defender has played for the Jets, and the y-axis is the following season.

Untitled_medium

A R2 of 0.0002 is essentially non-existent. You are likely to make a stronger correlation blindfolded and putting dots on there at random.

Well what about relative save percentage then?

Untitled2_medium

Again we see a pattern that mirrors randomness. It is almost 50% likely that a player who had a positive or negative effect will see the opposite the following season.

What does this mean for the Jets?

One: It means that in the new NHL true defending is making it so you don't have to defend.

The best defensive teams defend by limiting the other team's chances in taking attempts, not reducing the quality of attempts.

Darryl Sutter recently said this:

The game’s changed. They think there’s defending in today’s game. Nah, it’s how much you have the puck. Teams that play around in their own zone (say) they’re defending but they’re generally getting scored on or taking face-offs and they need a goalie to stand on his head if that’s the way they play

Two: It means that Dustin Byfuglien has been overly chastised for goals against that have occured.

I personally do believe that he is below average in the defensive zone, but less so than many make it out to be. It also means that Byfuglien makes up for his poor defensive abilities.

Dave Tippet once said this:

We had a player that was supposed to be a great, shut-down defenseman. He was supposedly the be-all, end-all of defensemen. But when you did a 10-game analysis of him, you found out he was defending all the time because he can’t move the puck. Then we had another guy, who supposedly couldn’t defend a lick. Well, he was defending only 20 percent of the time because he’s making good plays out of our end. He may not be the strongest defender, but he’s only doing it 20 percent of the time. So the equation works out better the other way. I ended up trading the other defenseman.

3: Never mind #BlameBuff, it really should be #BlamePavelec.

...There is a fourth note as well; Byfuglien this season experienced the lowest PDO of his career. While I'm sure not all GMs are dense enough to consider plus/minus in their evaluations, there is still a tendency for trade and contract value to largely swing with PDO fluctuations. This is primarily due to natural tendencies to notice direct contributions, as discussed earlier. Everyone knows that Byfuglien is probably the Jets best trade chip that is also movable. The emergence of Jacob Trouba has given the Jets three right shooting top 4 defensemen. This means that the Jets may not get full value for the defender.

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