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Download Date | 6/19/19 12:24 AM 86 | D J. Quant. Anal. Sports 2019; 15(2): 85–95 Daniel Daly-Grafstein* and Luke Bornn Rao-Blackwellizing field goal percentage https://doi.org/10.1515/jqas-2018-0064 percentage (eFG%). Shooting percentages play a large role in influencing both fan and coaching evaluation of play- Abstract: Shooting skill in the NBA is typically measured ers, and are often used to predict future player perfor- by field goal percentage (FG%) – the number of makes out mance when making decisions regarding free agency or of the total number of shots. Even more advanced met- draft selection. rics like true shooting percentage are calculated by count- Predicting a player’s FG% given past shooting is a dif- ing each player’s 2-point, 3-point, and free throw makes ficult task. Shooting percentages are highly variable, espe- and misses, ignoring the spatiotemporal data now avail- cially on longer shots like 3-point attempts. For example, it able (Kubatko et al. 2007). In this paper we aim to better takes roughly 750 3-point attempts before a player’s shoot- characterize player shooting skill by introducing a new ing percentage stabilizes, where over half of the variation estimator based on post-shot release shot-make probabil- in their 3-point percentage (3P%) is explained by shoot- ities. Via the Rao-Blackwell theorem, we propose a shot- ing skill, rather than noise (Blackport 2014). Additionally, make probability model that conditions probability esti- 3P% has been shown to be an unreliable metric in terms mates on shot trajectory information, thereby reducing the of its ability to discriminate between players and its stabil- variance of the new estimator relative to standard FG%. ity from one season to the next (Franks et al. 2016). As the We obtain shooting information by using optical tracking proportion of shot attempts taken as 3-pointers increases, data to estimate three factors for each shot: entry angle, with total attempts having risen nearly 50% over the last shot depth, and left-right accuracy. Next we use these fac- 8 years (Young 2016), overall FG% becomes more variable tors to model shot-make probabilities for all shots in the and less stable. 2014–2015 season, and use these probabilities to produce Part of the large variation in shooting percentages a Rao-Blackwellized FG% estimator (RB-FG%) for each is likely due to the many contextual factors that con- player. We demonstrate that RB-FG% is better than raw tribute to the probability of a shot make. Improvements FG% at predicting 3-point shooting and true-shooting per- to FG% prediction have been made by including some of centages. Overall, we find that conditioning shot-make these covariates in shot-make prediction models (Piette, probabilities on spatial trajectory information stabilizes Sathyanarayan, and Zhang 2010; Cen et al. 2015). How- inference of FG%, creating the potential to estimate shoot- ever, because of the small differences that separate the ing statistics earlier in a season than was previously true shooting skill of players in the NBA, chance varia- possible. tion may also contribute significantly to the variation and Keywords: basketball; bayesian regression; optical instability of FG%. Optical tracking data of shot trajecto- tracking; shot trajectories; variance reduction ries can potentially reduce noise in shooting metrics by allowing us to differentiate shots that rim out, air balls, and (unintentional) banks, giving us more information 1 Introduction about players’ shooting skill with fewer shots. This idea has been demonstrated recently during practice shoot- Field goal percentage is a common measure of shooting ing sessions, where FG% augmented by precise shot fac- skill and efficiency in the National Basketball Associa- tor information gathered during these sessions improved tion (NBA), and general shooting prowess is often defined the prediction of future shooting (Marty and Lucey 2017; for players by their overall FG%. It can be used in its Marty 2018). Accurate estimates of shot factors using opti- raw form, or as a component of more advanced metrics cal tracking data from live games may allow for a sim- like true-shooting percentage (TS%) or effective field goal ilar improvement in the prediction of in-game shooting metrics. In this paper, we seek to reduce the variation in pre- *Corresponding author: Daniel Daly-Grafstein, Department of dicting player FG% using NBA optical tracking data. We Statistics, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada, e-mail: [email protected] begin the paper by introducing a new estimator for FG%, Luke Bornn: Department of Statistics, Simon Fraser University, RB-FG%, based on aggregating shot-make probabilities. Burnaby, British Columbia V5A 1S6, Canada, e-mail: [email protected] Estimation of shot-make probabilities is then split into Unauthenticated Download Date | 6/19/19 12:24 AM 86 | D. Daly-Grafstein and L. Bornn: Rao-Blackwellizing field goal percentage two main parts. First, using spatio-temporal information Therefore, inference for θ is the same under the provided by the tracking data, we model shot trajectories Bernoulli and expected Beta-Binomial distributions. in order to estimate the depth, left-right distance, and Furthermore, suppose we obtain Xi (make or miss) and entry angle of balls entering the basket. Next, we use a pi (the probability that shot i will go in). Let Π(Xi , pijθ, v) regression model to estimate the probability of each shot be the joint distribution of Xi and pi. It follows that: going in. We define the average of these estimated prob- abilties, RB-FG%, as our new estimator of FG% for each Π(Xi , pijθ, v) = Π(Xijpi)Π(pijθ, v) player. Finally, we compare the predictive ability of the RB- where Π(Xijpi) and Π(pijθ, v) are the Bernoulli and Beta FG% estimator to its raw counterpart that does not utilize distributions, respectively. Consequently we have that trajectory information. given pi, Xi is independent of θ. Thus pi is sufficient for θ. Now let θ^ be the raw FG% estimate and θ^RB be the RB-FG% estimate. We have: 2 The Rao-Blackwellized N 1 X θ^ = Xi (FG%) estimator N i=1 In this section we introduce our new estimator for FG% N (︀ )︀ 1 X based on shot-make probabilities. When trying to predict θ^ = E θ^jp , ... , pN = pi (RB-FG%) RB 1 N a player’s future FG% using their past FG%, each shot i=1 Xi is treated as Bernoulli random variable with proba- Thus the RB-FG% is simply the conditional expecta- bility of success θ, where θ is a measure of the player’s tion of raw FG% given these shot-make probabilities pi. true FG%. However, shot trajectories provided by optical Because under the Beta-Binomial model pi is sufficient for tracking data give us more information for each shot than θ, by the Rao-Blackwell Theorem we have: simply whether it is a make or a miss. Incorporating this information into a shot model may allow us to reduce the (︀ )︀ (︀ )︀ MSE θ^RB ≤ MSE θ^ variance involved in estimating and predicting shooting skill. Therefore, we can define an alternative model where Unfortunately, we are unable to know the true prob- the probability of a shot-make varies depending on its ability that a shot will go in. Therefore, as decribed trajectory, and shots are modeled as Beta-Bernoulli ran- below, we will use estimates of shot-make probabilities dom variables Xi ∼ Bern(pi) with pi ∼ Beta(θv, (1−θ)v), based on shot trajectory information to obtain an esti- θ where again is the true FG% of a player, defining their mate of RB-FG%. Using an estimate of θ^RB means that corresponding Beta distribution of shot-make probabil- the inequality above does not necessarily hold. How- ties. Each player’s shooting ability is now modeled by a ever, as we will see in Section 4, our estimates of shot- Beta distribution, and the probability of a shot going in fol- make probabilites are accurate and precise enough that p p lows a Bernoulli distribution indexed by i, where i is a this estimate of θ^RB still leads to a decrease in vari- draw from that player’s Beta distribution. ance and prediction error relative to raw FG%. For sim- As shown below, inference under the model in which plicity, moving forward we will refer to the RB-FG% shots are treated as Bernoulli random variables and infer- estimator based on estimated shot-make probabilities ence under the expected Beta-Binomial of our new model as θ^RB. is the same (Skellam 1948). Let Π(Xijθ, v) be the likelihood of the expected Beta-Binomial distribution, i.e. the likeli- hood of the Beta-Binomial distribution if you mariginalize 3 Estimating shot-make out the pi’s, and let B(·) be the beta function. probabilities 1 Z Xi+θv−1 (1−θ)v−Xi pi (1 − pi) Π(Xijθ, v) / dpi B(θv, (1 − θ)v) 3.1 Measuring shot factors 0 B X + θv − X + − θ v In order to estimate shot-make probabilities, we first mea- / ( i , 1 i (1 ) ) B(θv, (1 − θ)v) sure three shot factors based on how each shot entered the basket – left-right accuracy, depth, and entry angle – fol- X −X / θ i (1 − θ)(1 i) lowing the procedure of Marty and Lucey (2017). We define Unauthenticated Download Date | 6/19/19 12:24 AM D. Daly-Grafstein and L. Bornn: Rao-Blackwellizing field goal percentage | 87 left-right accuracy as the deviation of the ball from the the shot in the tracking data, we use a quadratic polyno- centre of the hoop as the ball crosses the plane of the bas- mial to model the height, and estimate the coefficients by ket (Figure 1A).
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