NBA historians will remember the summer of 2017 for its unprecedented movement of bona fide stars: Jimmy Butler, Gordon Hayward, Chris Paul, Paul George, Kyrie Irving, Isaiah Thomas, Carmelo Anthony, and Paul Millsap will don new colors as the season kicks off this week. Many of these moves were driven by the ever-increasing “rings or bust” approach to NBA team-building, where teams either stockpile superstars to chase championships, or they crater to the bottom of the standings to build their team through top draft picks. Along these same lines, they are less inclined to use roster spots and cap room to retain middling stalworth veterans, instead opting to take fliers on high-variance youngsters. The combination of these factors means that continuity–keeping the same group of players together from one year to the next–is increasingly scarce.

Intuitively, the value of continuity makes sense: a team with collective reps from prior years learns to jell into a sum that is greater than its parts. But how can you quantify continuity? How many additional wins can a low-turnover team expect? Based on the past 20 years of roster turnover and performance, continuity does matter, but less than you might think.

To approach this analysis, I set out to answer the following question:

Given the quality of a roster, how much does continuity influence team success?

First, to measure the quality of a roster, I needed to measure the independent quality of each player. I used Win Shares, which divvies up a team’s success to its players. While many other metrics could have been used , I chose Win Shares because I wanted to approximate how many wins any arbitrary assembly of players would be expected to win, regardless of who they played for the prior year.

Next, I defined continuity as the percent of a team’s Win Shares that came from players who were on the team last year. For example, Oklahoma City’s current roster had 46.7 Win Shares in the 2016-17 season, 29.7 of which came from players who were on OKC last year. So, OKC’s Continuity for the 2017-18 season is 29.7/46.7 = 64%.

Another advantage of using Win Shares is that it accounts for each player’s importance to a team when he stays, as well as the disruptive impact of bringing in new players. In other words, Russell Westbrook staying with OKC is important, while Kyle Singler staying is…ahem…less so. Similarly, incorporating Paul George and Carmelo Anthony has a greater impact than incorporating Raymond Felton.

Using this measure of continuity, I looked over the past 20 years and measured its correlation with Win Percentage :

I quickly realized that this is a chicken-and-egg problem: good teams keep their cores, while lousy teams slough their scrubs. Who’s to say if continuity is the cause or the effect? Therefore, identifying this correlation is not sufficient for proving causality.

Let’s revisit our key question:

Given the quality of a roster, how much does continuity influence team success?

So, my next challenge was to control for the quality of each roster, independent of continuity. I did this by summing each of their players’ Win Shares from the prior year. While this glazes over some key details (rookies are not considered, and there is no adjustment in year-over-year changes in player productivity), it strongly correlates with actual performance :

By subtracting Last Year Win Shares from This Year Win Percentage, I could identify over-performing (positive) and under-performing (negative) teams. The final question was: how does continuity correlate with this deviation in performance?

While the relationship is tenuous, there is actually a slight positive correlation . This means that continuity explains some part of the deviation, albeit a small one.

Extrapolating this relationship onto the 2017-18 season, we can calculate the expected Wins for this season:

Ultimately, we see that continuity’s impact is not that great; 1.6 Wins separates the team with the most continuity (Portland) from the team with the least (Boston).

This analysis should be taken with a grain of salt. As previously noted, using Last Year Win Shares to project This Year Win Percentage makes many assumptions, and some of the results just don’t make sense. (The Celtics will definitely win more than 39 games this year, and the Wolves are not winning 59.) Also, the correlation between continuity and deviation from the expected performance is a weak one, which helps explain why the measurable impact of continuity is so small.

With that said, we can still apply these learnings to today’s NBA. In particular, it is clear that adding a superstar to a team dwarfs any concern of discontinuity. Whether it was the Celtics adding Kevin Garnett and Ray Allen in 2007, the Heat luring LeBron James and Chris Bosh in 2010, or the Warriors nabbing Kevin Durant in 2016, each team was unequivocally better off, despite the challenges of incorporating new players of such high importance. This season, the same will go for the Celtics, Thunder, Rockets, Wolves, and Nuggets.

Instead, continuity should be considered more with regards to veteran rotation players. Maintaining a steady cast of role players builds team identity and drives the benefits of continuity. Any successful team has that one or more steady veteran to rely on: Derek Fisher on the early- and late-2000’s Lakers, Tony Allen and Kendrick Perkins on the late-2000’s Celtics, Udonis Haslem on the early-2010’s Heat, or Andre Iguodala on the mid-2010’s Warriors, just to name a few.

In the increasingly competitive arms race of NBA team-building, teams need any help they can get in order to make it to the top. A small boost from continuity could be the difference between going home early and raising the Larry O’Brien Trophy.

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