Yesterday for the first time, I took the playoff game between Cleveland and Toronto as an opportunity to test out a script I wrote in R that keeps track of key statistics during a game in real time (well, every 30 seconds). Based on previous work, it is evident that championship-calibre teams are the ones that have excellent 2PT-FG% and the ability to draw fouls, so I tracked these during the game, and I came up with the following plot of several time series:
One sees for example that while Toronto started off the game with a much higher 2PT FG%, towards the end Cleveland ended up winning that battle.
A video of this animation is as follows (set the YouTube player to 1080p + FullScreen for Max Quality!)
An interesting question to ask is how are these series correlated? Well, let’s see:
One sees immediately from the correlation plot above that there is a very strong correlation between Cleveland’s point difference and Toronto’s personal fouls, with some strong correlations attributed to Cleveland’s 2-Point FG% as well. The equal and opposite is true for Toronto’s point difference. It seems that during a game of this intensity in the playoffs, drawing fouls is a very important factor in determining which team leads and eventually wins in the game combined with 2-Point field goal percentage.
I’ve been interested for some time on figuring out an analytical way to determine what characterizes an NBA team as a playoff team. Looking at the previous six seasons, I pulled together almost 65 different statistics that characterize how a team plays, and then performed a classification tree analysis. I found the following result:
For the above tree, the misclassification error rate was 2.73%. Also, MOV stands for margin of victory, o3PA is the number of opponent three-point attempts per game, DRtg, is defensive rating, which is the number of points a team allows per 100 possessions, and so on. The data itself was taken from Basketball-Reference.com.
We see that the following patterns emerge among NBA playoff teams over the past number of seasons.
We asked, “Which player will be better in 2015-16?” To decide, voters had to consider both the quality and quantity of each player’s contributions to his team’s ability to win games. More than 100 voters weighed in on nearly 30,000 pairs of players.
Of course, while I suspect this type of thing has to be just for fun , it has generated a great deal of controversy with many arguments ensuing between fans. For example, Kobe Bryant being ranked 93rd overall in the NBA this year gained a fair deal of criticism from Stephen A. Smith on ESPN First Take.
In general, at least to me, it does not make any sense to rank players from different positions that bring different strengths to a team sport such as basketball. That is, what does it really mean for Tim Duncan to be better than Russell Westbrook (or vice-versa), or Kevin Love to be better than Mike Conley (or vice-versa), etc…
From a mathematical/data science perspective, the only sensible thing to do is to take all the players in the league, and apply a clustering algorithm such as K-means clustering to group players of similar talents and contributions into groups. This is not a trivial thing to do, but it is the sort of thing that data scientists do all the time! For this analysis, I went to Basketball-Reference.com, and pulled out last season’s (2014-2015) per game averages of every player in the league, looking at 25 statistical factors from FGA, FG% to STL, BLK, and TOV. One can see that this is a 25-dimensional problem.
Our goal then is to consider the problem where denoting as sets containing the observations in each cluster, we want to solve the optimization problem:
where is our distance measure. We use the squared Euclidean distance to define the within-cluster variation, and then solve:
The first thing to do is to decide how many clusters we want to use in our solution. This is done by looking at the within sum of squares (WSS) plot:
First, we will use 3 clusters in our K-means solution. In this case, the between sum of squares versus total sum of squares ratio was 77.0%, indicating a good “fit”). We use three clusters to begin with, because based on visual inspection, the data clusters very nicely into 3 clusters. The plots obtained were as follows:
The three clusters of players can be found in the following PDF File. Note that the blue circles represent Cluster 1, the red circles represent Cluster 2, and the green circles represent Cluster 3.
Next, we dramatically increase the number of clusters to 20 in our K-means solution.
Performing the K-means clustering, we obtain the following sets of scatter plots. (Note that, it is a bit difficult to display a 25×25 plot on here, so I have split them into a series of plots. Note also, that the between sum of squares versus total sum of squares ratio was 94.8 %, indicating a good “fit”):
The cluster behaviour can be seen more clearly in three dimensions. We now display some examples:
The 20 groups of players we obtained can be seen in the PDF file linked below:
Two sample group clusters from our analysis are displayed below in the table. It is interesting that the analysis/algorithm provided that Carmelo Anthony and Kobe Bryant belong in one group/cluster while LaMarcus Aldridge, Lebron James, and Dwyane Wade belong in another cluster.
If we use more clusters, players will obviously be placed into smaller groups. The following clustering results can be seen in the linked PDF files.
I did not include the visualizations for these computations because they are quite difficult to visualize.
Looking at the 100 Clusters file, we see two interesting results:
In Cluster 16, we have: Carmelo Anthony, Chris Bosh, Kobe Bryant and Kevin Martin
In Cluster 74, we have: LaMarcus Aldridge, Anthony Davis, Rudy Gay, Blake Griffin, LeBron James and Russell Westbrook
We therefore see that is does not make much mathematical/statistical sense to compare and two pairs of players. In my opinion, the only logical thing to do when ranking players is to decide on rankings within clusters. So, based on the above analysis, it makes sense to ask for example whether Carmelo is a better player than Kobe or whether Lebron is a better player than Westbrook, etc… But, based on last season’s statistics, it doesn’t make much sense to ask whether Kobe is a better player than Westbrook, because they have been clustered differently. I think ESPN could benefit tremendously by using a rigorous approach to these sorts of things which spark many conversations because many people take them seriously.
The purpose of this post is to determine whether basketball teams who choose to employ an offensive strategy that involves predominantly shooting three point shots is stable and optimal. We employ a game-theoretical approach using techniques from dynamical systems theory to show that taking more three point shots to a point where an offensive strategy is dependent on predominantly shooting threes is not necessarily optimal, and depends on a combination of payoff constraints, where one can establish conditions via the global stability of equilibrium points in addition to Nash equilibria where a predominant two-point offensive strategy would be optimal as well. We perform a detailed fixed-points analysis to establish the local stability of a given offensive strategy. We finally prove the existence of Nash equilibria via global stability techniques via the monotonicity principle. We believe that this work demonstrates that the concept that teams should attempt more three-point shots because a three-point shot is worth more than a two-point shot is therefore, a highly ambiguous statement.
We are currently living in the age of analytics in professional sports, with a strong trend of their use developing in professional basketball. Indeed, perhaps, one of the most discussed results to come out of the analytics era thus far is the claim that teams should shoot as many three-point shots as possible, largely because, three-point shots are worth more than two-point shots, and this somehow is indicative of a very efficient offense. These ideas were mentioned for example by Alex Rucker who said “When you ask coaches what’s better between a 28 percent three-point shot and a 42 percent midrange shot, they’ll say the 42 percent shot. And that’s objectively false. It’s wrong. If LeBron James just jacked a three on every single possession, that’d be an exceptionally good offense. That’s a conversation we’ve had with our coaching staff, and let’s just say they don’t support that approach.” It was also claimed in the same article that “The analytics team is unanimous, and rather emphatic, that every team should shoot more 3s including the Raptors and even the Rockets, who are on pace to break the NBA record for most 3-point attempts in a season.” These assertions were repeated here. In an article by John Schuhmann, it was claimed that “It’s simple math. A made three is worth 1.5 times a made two. So you don’t have to be a great 3-point shooter to make those shots worth a lot more than a jumper from inside the arc. In fact, if you’re not shooting a layup, you might as well be beyond the 3-point line. Last season, the league made 39.4 percent of shots between the restricted area and the arc, for a value of 0.79 points per shot. It made 36.0 percent of threes, for a value of 1.08 points per shot.” The purpose of this paper is to determine whether basketball teams who choose to employ an offensive strategy that involves predominantly shooting three point shots is stable and optimal. We will employ a game-theoretical approach using techniques from dynamical systems theory to show that taking more three point shots to a point where an offensive strategy is dependent on predominantly shooting threes is not necessarily optimal, and depends on a combination of payoff constraints, where one can establish conditions via the global stability of equilibrium points in addition to Nash equilibria where a predominant two-point offensive strategy would be optimal as well. (Article research and other statistics provided by: Hargun Singh Kohli)
2. The Dynamical Equations
For our model, we consider two types of NBA teams. The first type are teams that employ two point shots as the predominant part of their offensive strategy, while the other type consists of teams that employ three-point shots as the predominant part of their offensive strategy. There are therefore two predominant strategies, which we will denote as , such that we define
We then let represent the number of teams using , such that the total number of teams in the league is given by
which implies that the proportion of teams using strategy is given by
The state of the population of teams is then represented by . It can be shown that the proportions of individuals using a certain strategy change in time according to the following dynamical system
where we have defined the average payoff function as
Now, let represent the proportion of teams that predominantly shoot two-point shots, and let represent the proportion of teams that predominantly shoot three-point shots. Further, denoting the game action set to be , where represents a predominant two-point shot strategy, and represents a predominant three-point shot strategy. As such, we assign the following payoffs:
We therefore have that
From (6), we further have that
From Eq. (4) the dynamical system is then given by
subject to the constraint
Indeed, because of the constraint (10), the dynamical system is actually one-dimensional, which we write in terms of as
From Eq. (11), we immediately notice some things of importance. First, we are able to deduce just from the form of the equation what the invariant sets are. We note that for a dynamical system with flow , if we define a function such that , where , then, the subsets of defined by , and are invariant sets of the flow . Applying this notion to Eq. (11), one immediately sees that , , and are invariant sets of the corresponding flow. Further, there also exists a symmetry such that , which implies that without loss of generality, we can restrict our attention to .
3. Fixed-Points Analysis
With the dynamical system in hand, we are now in a position to perform a fixed-points analysis. There are precisely three fixed points, which are invariant manifolds and are given by:
Note that, actually contains and as special cases. Namely, when , , and when , . We will therefore just analyze, the stability of . represents a state of the population where all teams predominantly shoot three-point shots. Similarly, represents a state of the population where all teams predominantly shoot two-point shots, We additionally restrict
which implies the following conditions on the payoffs:
With respect to a stability analysis of , we note the following. The point is a: • Local sink if: , • Source if: , • Saddle: if: , or .
What this last calculation shows is that the condition which always corresponds to the point , which corresponds to a dominant 3-point strategy always exists as a saddle point! That is, there will NEVER be a league that dominantly adopts a three-point strategy, at best, some teams will go towards a 3-point strategy, and others will not irrespective of what the analytics people say. This also shows that a team's basketball strategy really should depend on its respective payoffs, and not current "trends". This behaviour is displayed in the following plot.
Further, the system exhibits some bifurcations as well. In the neigbourhood of , the linearized system takes the form
Therefore, destabilizes the system at . Similarly, destabilizes the system at . Therefore, bifurcations of the system occur on the lines and in the four-dimensional parameter space.
4. Global Stability and The Existence of Nash Equilibria
With the preceding fixed-points analysis completed, we are now interested in determining global stability conditions. The main motivation is to determine the existence of any Nash equilibria that occur for this game via the following theorem: If is an asymptotically stable fixed point, then the symmetric strategy pair , with is a Nash equilibrium. We will primarily make use of the monotonicity principle, which says let be a flow on with an invariant set. Let be a function whose range is the interval , where , and . If is decreasing on orbits in , then for all ,
Consider the function
Then, we have that
For the invariant set , we have that . One can then immediately see that in ,
Therefore, by the monotonicity principle,
Note that the conditions and correspond to above. In particular, for , , which implies that is globally stable. Therefore, under these conditions, the symmetric strategy is a Nash equilibrium. Now, consider the function
We can therefore see that
Clearly, in if for example and . Then, by the monotonicity principle, we obtain that
Note that the conditions and correspond to above. In particular, for , , which implies that is globally stable. Therefore, under these conditions, the symmetric strategy is a Nash equilibrium. In summary, we have just shown that for the specific case where and , the strategy is a Nash equilibrium. On the other hand, for the specific case where and , the strategy is a Nash equilibrium. 5. Discussion In the previous section which describes global results, we first concluded that for the case where and , the strategy is a Nash equilibrium. The relevance of this is as follows. The condition on the payoffs thus requires that
That is, given the strategy adopted by the other team, neither team could increase their payoff by adopting another strategy if and only if the condition in (23) is satisfied. Given these conditions, if one team has a predominant two-point strategy, it would be the other team’s best response to also use a predominant two-point strategy. We also concluded that for the case where and , the strategy is a Nash equilibrium. The relevance of this is as follows. The condition on the payoffs thus requires that
That is, given the strategy adopted by the other team, neither team could increase their payoff by adopting another strategy if and only if the condition in (24) is satisfied. Given these conditions, if one team has a predominant three-point strategy, it would be the other team’s best response to also use a predominant three-point strategy. Further, we also showed that is globally stable under the conditions in (23). That is, if these conditions hold, every team in the NBA will eventually adopt an offensive strategy predominantly consisting of two-point shots. The conditions in (24) were shown to imply that the point is globally stable. This means that if these conditions now hold, every team in the NBA will eventually adopt an offensive strategy predominantly consisting of three-point shots. We also provided through a careful stability analysis of the fixed points criteria for the local stability of strategies. For example, we showed that a predominant three-point strategy is locally stable if , while it is unstable if . In addition, a predominant two-point strategy was found to be locally stable when , and unstable when . There is also they key point of which one of these strategies has the highest probability of being executed. We know that
That is, the payoff to a team using strategy in a league with profile is proportional to the probability of this team using strategy . We therefore see that a team’s optimal strategy would be that for which they could maximize their payoff, that is, for which is a maximum, while keeping in mind the strategy of the other team, hence, the existence of Nash equilibria. Hopefully, this work also shows that the concept that teams should attempt more three-point shots because a three-point shot is worth more than a two-point shot is a highly ambiguous statement. In actuality, one needs to analyze what offensive strategy is optimal which is constrained by a particular set of payoffs.
The NBA finals are exactly five days away, and I wanted to present an analysis breaking down the matchup between The Golden State Warriors and Cleveland Cavaliers.
I used machine and statistical learning techniques to generate the most probable scenarios for the outcome of each game, and this is what I found.
Note that the probabilities listed above are not theprobabilities for a team to win a specific game, they are the probabilities of a specific scenario occurring. Also, multiple scenarios can occur in a single game, so the probability of multiple scenarios occurring would be the sum of the individual ones.
The Model Results So Far (Updated: June 11, 2015)
Game 1: Scenario Outcomes: 1 and 2 – GSW win
Game 2: Scenario Outcome: 9 – CLE win
Game 3: Scenario Outcomes: 5, 8 – CLE win
Thoughts so far: Despite GSW being down right now 2-1, I still believe that Cleveland’s wins were statistical anomalies. Cleveland’s Game 2 and Game 3 wins according to our model only had 1.07%, 9.34%, and 1.765% chances of occurring in this series. Whereas, the GSW Game 1 win had a 44% chance of occurring in this series.
Game 4: Scenario Outcome: 2 – GSW win
Updated: June 14, 2015
Game 5: Scenario Outcomes: 1,2 – GSW win
Thoughts: All of GSW wins have been the dominant scenarios in this series, i.e., Outcomes 1 and 2. All of CLE wins in this series have been statistical anomalies/outliers. This pattern continued in Game 5.
Updated: June 17, 2015
Game 6: Scenario Outcomes: 1,2 – GSW win
Another GSW win through the dominant scenarios in the series, as expected.
Some controversy was stirred up today when Knicks President and Basketball coaching legend Phil Jackson made the following tweets regarding three-point shooting teams not doing so well in the second round of the playoffs:
NBA analysts give me some diagnostics on how 3pt oriented teams are faring this playoffs…seriously, how's it goink?
I am testing out a new algorithm that I have been developing over the past few months that attempts to predict the outcome of sports games, in particular, NBA games. I am taking it out for a “Test Run” today. Here is what I predict:
Probabilities in principle are not too difficult to predict assuming you have the correct algorithm! What is more challenging is trying to predict the scores. Here is my prediction for the individual game outcomes:
Note: p1 and p2 denote probabilities of each team winning.