A New Way to Measure Turnovers Using SportVU
USA Today Sports
There is no controversy that turnovers are bad. They are probably the pet peeve of every coach, as by definition, it’s a wasted possession, even more than a very badly chosen shot.
There is also a widespread recognition that turnovers by a player need to be scaled to the opportunity to commit turnovers. Just as scoring scoring twenty points on twenty-five possessions is not as valuable as scoring eighteen points on fourteen possessions, a starting point guard with four turnovers in thirty-two minutes is less egregious than a back up center letting the only four passes near him go through his hands.
The two most popular ways to scale turnovers are assist to turnover ratio and turnover percentage. Assist to Turnover ratio is the number of assists a player is credited with divided by the number of turnovers the player is charged with committing. Players who make more assists generally have the ball in their hands more and errant passes are one source of turnovers.
Turnover percentage is generally considered the most advanced way to scale turnovers and is calculated as percentage of the possessions a player ‘finishes.’ The formula is effectively turnovers as a percentage of shots and trips to the free throw line.
SportVU Player Tracking data gives us another way to scale turnovers, with player touches of the basketball data. The SportVU touch data tracks and counts every time a player touches the ball as well as the time that he has possession of the ball. I matched the SportVu data from the NBA website with the turnover count from their standard data for each player then calculated the number of turnovers each player committed per 100 touches.
Of players with over 150 minutes of playing time, Utah rookie Rudy Gobert tops both the turn over per 100 touches list and the TOV% list. The top twenty offenders are listed below, with those also appearing in the top twenty based on TOV% highlighted in green.
The Falsely Accused
Comparing the different measures overall, TOV% and TOV per 100 touches correlate pretty well with an R^2 of around 0.50 for players with over 150 minutes of playing time. However, there are what look like systematic differences. The turnovers by low-usage players are highlighted while gunners’ turnovers get de-emphasized.
The table below has the players with the biggest fall in turn overs when switching from TOV% to TOV per 100 touches. Both measures were standardized based on the standard deviation, then the standardized measures were compared. Players that do not shoot often, many of them big men, get tagged with a higher turnover rate using TOV%.
Kendrick Perkins by TOV/100 touches turns the ball over at a slightly above average rate, but because of his low usage rate they are magnified to 25% of his possessions, 1.7 standard deviations above average. Players like Jan Vesely and Omer Asik go from approximately average fumblers to significant ones. While players like Nick Collison and DeAndre Jordan go from below average offenders per touch to above average turn overs using TOV%.
Getting Away With It
On the other side of the equation some players with a quick trigger finger may have had their turn over propensities ignored to some extent. The list of players with the largest increase in their turnover propensity is mostly high usage players including old stats friend Rudy Gay at third on the list. Gay turns over the ball on 13.2% of the possessions he finishes, but because he finishes so many possessions by throwing the ball toward the hoop that may credit him with better ball handling than he has. He turns the ball over 6.3 times per 100 touches, well above the league average of 3.8.
Klay Thompson actually shows the largest increase in turn over propensity, followed by high usage players like Dion Waiters, Gay, and DeMarcus Cousins.
Briefly, assist to turnover ratio doesn’t come out looking great as measure judging by TOV /100 touches. The two are not well correlated. Because assists are so position related, most of the players that look better by Assist/TO are point guards, lead by Chris Paul, and most of the players that look worse by it are front court players, lead by Rudy Gobert (who doesn’t need anyone to make his turnovers look worse) and Ryan Hollins. Because of the positional bias assist to turnover ratio is probably only helpful comparing players at the same position or role.
Turnover percentage will, and probably, should still continue to be used as the advanced turn over metric given that it measures a player’s tendencies for turn overs fairly well and we have over thirty years of data using that measure for any league where basketball is played. That gives us a good base for both analysis and bar stool discussions, but having turnovers per touch gives us a new level of testing especially for players with very different offensive roles.
*All data via NBA.com as of 12-30-2013