UNDERSTANDING GVT
(Part 1 – Aug 2009)
while offensive production is relatively easy to measure, the defensive contributions of a player are less so.
In addition, it is almost impossible to compare players of different positions,
GVT (Goals Versus Threshold) was my personal attempt to address these problems.
GVT is the value of a player, in goals, above what a replacement player would have contributed.
players’ contributions always come down to scoring goals and preventing them.
Each player’s role, no matter his position, is to try and increase the goal differential in favor of his team.
Using this standard, all players can be compared by the same yardstick: how much did they help (or harm) their team’s goal differential?
Here are the five fundamental characteristics of GVT that would also be needed as the building blocks for any other sort of all encompassing-like statistic in hockey:
• GVT is measured in goals. This makes it a convenient unit that hockey fans are already comfortable with.
• GVT compares hockey players of all positions and over any period of time.
• GVT only uses statistics that lead directly to goals.
• GVT has built-in accounting. The sum of player GVTs on a team equals that team’s GVT plus the replacement level. This makes it much easier to measure “how good would this team be replacing player A with player B?” It is also essential in that player success is correlated with team success, which after all is the entire point of the sport.
• GVT automatically normalizes for the strength of the league.
To calculate GVT, I have split hockey players’ responsibilities up into 3 functions: offense, defense, and goaltending.
I have attempted to follow conventional hockey wisdom and assigned to goals a value of 1.5 assists.
The second function is defensive: I have defined the defensive responsibility as preventing shots on goal.
A player’s defensive value can be measured by two statistics: the number of shots on goal that his team allows, and his plus-minus compared to his team’s even-strength goal differential.
Also, as an attempt at mirroring conventional hockey wisdom, I have assumed that each defenseman has twice the defensive responsibilities that a forward has.
The third function is goaltending: blocking the shots that do get taken on goal.
Ultimately, that is a goaltender’s only responsibility.
(Part 2 – Aug 2009)
Limitations of GVT:
GVT works best for regular season statistics.
Another problem is that, given the large number of games in the regular season, goal differential is a good proxy for overall success. In the playoffs, this is not as true
GVT does not measure a player's talent. The statistic measures a player's contribution to his team's goal differential.
It also does not take into account environment: a player will score more with better linemates, and I make no attempt to adjust for that. Other metrics used at Puck Prospectus, especially QUALTEAM and QUALCOMP, do a good job of that.
The Method
A player's GVT value is the sum of three things: his Offensive Goals Versus Average (OGVT), his Defensive Goals Versus Threshold (DGVT), and his Goaltending Goals Versus Threshold (GGVT). In recent years, with the introduction of the shootout in the NHL, a fourth component, Shootout Goals Versus Threshold (SGVT), has been added. Each of these factors is calculated independently. However, before calculating any GVT values, we must first estimate ice time.
Ice Time
Different players don’t spend the same amount of time on the ice, and consequently don’t get the same level of opportunity to contribute. To estimate a player’s contribution, it is important that we have a rough idea of his ice time.
CALCULATING GVT.
(Part 3 – Aug 2009)
Offensive GVT
Offensive contribtutions are well-measured statistically, using goals and assists. In order to arrive at a single value for offense, we must combine goals and assist in some manner.
the GVT method assumes that a goal is worth 1.5 times an assist. Using that, we calculate the team goal value of goals and assists
Goaltending GVT
A goaltender's contribution to his team's efforts to win is attempting to stop the shots that come his way. This is best measured by save percentage, which indicates the proportion of a goaltender's shots faced that are stopped.
A goaltender's GGVTraw is therefore calculated as the number of saves he made, less the number of saves that a goalie with a Threshold save percentage would have made
In practice, a goaltender’s save percentage is not uniquely due to his own caliber of play: some of it is due to the quality of the shots he faces.
Therefore, I attribute 75% of the responsibility of stopping a shot to the goaltender and 25% of it to the team’s defense.
Defensive GVT
The defense's contribution must be in preventing shots from being taken (plus the residual credit from the goaltending calculation). Therefore we use team shots allowed to assign value to team defense.
The responsibility of defense is not equal among all players. Specifically, defensemen are more responsible for defense than are forwards, just as forwards are more responsible for offense than are defensemen.
individual forwards are responsible for twice the offense of individual defensemen.
It follows that individual defensemen are probably twice as responsible as forwards for defense, since there is no reason to believe either players of either position are more valuable on the whole.
here, we move on to the second factor of DGVT, which differentiates players on a team based on their performance. For this, we need an indicator of individual defensive performance.
Unfortunately, this is the aspect of hockey that has the least useful statistical information. Plus-minus is the best mainstream statistic in this regard, and even it is only half defensive.
There are a variety of other problems with plus-minus as well, but it is the best statistic available for this purpose that has been tracked over a long period of time.
We can significantly improve plus-minus by adjusting it to account for team performance;
We therefore adjust a player's plus-minus based on the average plus-minus for his team
Shootout GVT
As shootout goals do count in the standings, it is important that we factor in players’ shootout contributions in the total value.
Putting It All TogetherCombining all four variables, a player's total Goals Versus Threshold figure is as follows:GVT = OGVT + DGVT + GGVT + SGVT
Comments