Sunday, November 1, 2015

How Good are NBA General Managers?

In a press conference this week, Lebron James talked about the 76ers' team-building process as they enter a third season with a substandard roster. He said, "It's always a process...You got to build things from the ground up. This year, it's about making a transition." This week, we ask, can we quantify this team-building process?

We've looked at metrics quantifying the clutchness of individual NBA players before, but what about from a team perspective? Is there a way to analyze the quality of a general manager, in making decisions about team selection and player acquisition?

This week, we aim to aquantify team chemistry - how well the players work together as opposed to individually. Then, we map out team chemistry in relation to player retention to see whether GMs can identify when they have good core teams or when they need to mix it up.

To quantify team chemistry, we compared the expected win share of each team (totaled individual win shares of each player on the roster) with the realized win share. We had to calculate expected win share per player as a point of comparison, because actual win shares are already impacted by team chemistry - players will score more points if they are on a team that works well together. Expected win-share would give us a baseline for how players perform on a neutral team.

To calculate expected win share, we used a metric called a similarity score, which quantifies how similar two players' career trajectories are. We used 2013-14 data to calculate these similarity scores, and predicted each player's win-share by the win-shares of the 20 most similar players at this point in their career trajectory, weighted by their similarity score. By using these other players, we hoped to average out the team effect and isolate the individual effect.

By totaling up these expected win-shares and realized win shares for everyone on a team's roster, we could quantify the "chemistry", or the performance above individual expectation, a team had. Then, we plotted this chemistry metric against retention. Retention was also measured based on win share - we calculated the win-share total of a team in one year, then the percent of the win-share that would remain next year after some players were traded away.

Plotting retention against chemistry allows us to classify NBA franchises into four categories, as shown in the table above. Teams with low retention but above-average performance are indications of a newly formed core team, as newly acquired players have figured out how to work together in a short period of time. General managers with a high retention and above-average performance team have found a core group of players that work well together. In these two instances, GMs should not break up their rosters.

On the other side of the spectrum, low retention and below-average performance teams are usually in the rebuilding stage, and GMs should continue looking for a better roster. High retention and below-average performance franchises are signs of teams falling apart in their chemistry, and GMs should seriously consider making some key changes to their roster.

With this in mind, we plotted retention vs. chemistry for all NBA teams in the past 10 seasons. In a quick glance, you will see that the data point are widely scattered, suggesting that some general managers tailor their retention in response to their team's performance, and others do not. Particularly, GMs in the bottom right quadrant should be mixing up their teams due to poor team chemistry, but are not.

The Lakers Are Doing It Right

Retention vs. Performance Above Expectation for the Los Angeles Lakers
A closer look into the Los Angeles Lakers reveals a franchise that really knows how to handle good and bad times. After winning 3 consecutive championships in 2000-2002, the Lakers had a series of mediocre seasons, and reshuffled a lot of its players. After a surprisingly positive 2006 season where they pushed the Phoenix Suns to seven games in the playoffs despite plenty of transfers, they realized they have created a "newly formed core team" (top left in graph).  For the next 4 years, the Lakers kept their main players, resulting in another golden era (2007-2011) where they consistently performed better than expected and two NBA championships (top right in graph). But after a poor 2011-2012 season, where they fell tamely to the Thunders in the playoffs, they realized they were maintaining a poor team (bottom right).  As a result, they decided to rebuild once again, trading and drafting players with the hope of looking for a new core group that can become a championship team. As of 2015, they have yet to find one (bottom left), but the Lakers seem to be sticking to a strategy that has won them 16 championships.

The Knicks Are Not Doing it Right

Retention vs. Performance Above Expectation for the New York Knicks
A team that has not followed this philosophy is the New York Knicks. For many years, the Knicks have disappointed their fanbase, making the playoffs on very few occasions despite paying high salaries to top players like Carmelo Anthony and A'mare Stoudemire. The reason for this is their failure to break up their core group of players despite inconsistent and poor performances. From 2007-2010, NYK teams were situated in the bottom right, showcasing the franchises' reluctance to rebuild. Recently in 2014 and 2015, the team has also been in this quadrant, and most recently had their worst season in history (17-65). This franchise really needs to reshuffle their team in order to be compete for a championship once again.

Is Lebron Right about the 76ers?

Indeed, it seems that the 76ers are quantifiably in a "transition" process, as Lebron called it. Looking at the past three years, they've had performance below expectation, but also low retention rates, indicating that their GM is aware of the fact that he has to shuffle his roster up. This marks a strategy turnaround from years 2007-10, when they appeared to retain heavily in spite of their dismal performance.

What This Means for the 2015-2016 Season

Retention vs. Performance Above Expectation for 2014-2015 Season Teams
We can also use the data to identify teams that are likely to perform well this season. Looking at just the 2015 season, we see that the Warriors, Hawks, Grizzlies all sit firmly in the top right quadrant. For the upcoming season, these teams have a retention rate of at least 65%, suggesting that they are continuing to build on their core teams. However, the Spurs, despite a 90% retention rate last year, are only keeping 58% of this team this season after trading Cory Joseph, Tiago Splitter and Aron Baynes, their lowest retention in a decade. Thus it remains to be seen if the Spurs can truly perform this season despite their reputation as one of the more team-oriented franchises in the league.

Some may argue that acquisition decisions are largely due to luck, and it is very difficult to attribute player performances to the competence of general managers. While this is a valid point, we hope that this data will cast some insight not on whether teams got lucky with the decisions they made, but rather whether they took advantage of favorable situations, such as converting high potential teams (top left) into a high-quality cohesive group (top right).

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