As a brief review of Part 1, the overall player rating reflects the sum contribution of all game actions from a player. And generally, the player contribution consists the chance of scoring when the player is done with the ball; minus the chance of scoring when he first received the ball. The chance of scoring is defined as the expected xG of a possession at that moment.
It’s worth noting that these subcategories do not overlap with each other; an effort was made to avoid double-counting actions. But while these subcategories do not overlap, it’s possible for a player’s time on the ball to cover several categories with a single touch. For example, a player could intercept a pass, successfully dribble past a defender, advance the ball, and then make a long connecting pass to a teammate. This one touch would create value for Defense-Turnover, Movement (value created moving from take-on to the area where the ball was passed), and Pass Value categories. Most commonly though, a player’s touch will involvement movement plus passing or shot value.
Shot value is calculated as the chance of a shot scoring (independent of the goalkeeper) minus the zone value of where the shot occurred. For this rating, the three possible end results of a shot are (1) shot on target using the head, (2) shot on target using the feet/other, and (3) a missed or blocked shot. For missed shots, the end result is equals the standard shot xG. For shots on target, the end result is the percentage of shots on target that actually score from the same zone (based on all shots over three MLS seasons).
The shot value subcategory is the most important for a team, with the highest average value. Shot value attempts to isolate the value created solely from the act of shooting. In other words, if a player dribbles through three defenders and takes a shot, only the act of shooting shows up in the shot value.
The end-result-minus-start-value approach leads to some key differences from a typical evaluation of shot quality using standard xG. Most significantly, the location value is subtracted from the result. Removing the location value prevents double-counting the value added during the build-up. Additionally, this feature also moderates the “poacher” value. Generating high xG shots from an already valuable location is not as impressive as generating a similar chance of scoring from further away.
Another key distinction is the post-shot component. Goals and shots on target are treated equally in order to minimize goalkeeper effects. Shot accuracy is really all the shooter controls, and if we want to isolate a player’s contribution, it shouldn’t matter whether the keeper is Nick Rimando, or a backup making his first start. In an initial pilot study, this method led to more consistent values from year to year than including goals scored.
A final distinction, penalty kicks are not significant features of shot value. Most penalty kicks are scored, and there is not a lot of variability in the outcome. And the pk math for shot value follows suit. The zone value for a pk is the avg xG for a penalty. And a missed shot is the xG minus the zone value, which is zero. And a shot on target has a higher chance of scoring, but minimally so because a penalty already has a high chance at scoring. As a result, a pk goal adds about .03 xG equivalents. (Most of the value for pks is assigned to the foul leading to the pk). As a result, shot value is not likely to be inflated, or harmed, by players that take many penalty kicks for their team.
Overall, the shot values reward shot accuracy, and minimize receiving the ball close to goal. And the shot value subcategory tends to be consistent over time. For players with 10+ shots, the year to year correlation for Shot Value per 90 minutes is 0.837, which is a bit higher than the xG per 90 correlation for the same group.
2018 Shots Value leaders*, as of September 3rd: