Every year, several NBA franchises part ways with their head coach after a disappointing season in order to try to change their direction. A disappointing season could be many things, ranging from an abysmal year with a record low wins to a season in which a team came up just short. When a letdown of a season inevitably happens for a team, the first blame often goes to the coaching and management. Fans and owners of teams with talent will blame the fit of the players (management’s fault) or the head coach’s inability to devise a scheme for the players to succeed in. Fans and owners of teams without talent will blame the management’s faulty team building methods or try to make changes to the coaching staff in an attempt to get out of the basement of the league and improve. There have been a surprising 81 coaching changes within the last ten seasons, meaning there are about 8 coaching changes a year. That is over 25% of NBA teams. The instability of the NBA head coaching job gave me inspiration to look into how often a coaching change improves a team and how well new head coaches in the NBA fare.
In order to quantify the improvement a new head coach brings upon his team, I used net rating to measure the team’s strength before and after the coaching change. Net rating is a number which describes a team’s average point differential per 100 possessions. In addition to net rating, I also recorded whether the new head coach made the playoffs and how many years it took if he did. The sample for this study was every head coach hired from 2010 to 2019. There were 81 coaches in the sample. All but three NBA teams had a new head coach at least once within the last ten years. The Heat (Erik Spoelstra), Mavericks (Rick Carlisle), and Spurs (Gregg Popovich) were the only teams without a coaching change from 2010 to 2019, an impressive feat given the instability of the NBA head coaching job.
The first thing that I looked at for each new head coach is the net rating of their team the prior season. I looked at this in order to get an idea of how talented the team was before the new coach took over. The average net rating of the season before a new head coach was -2.316, a net rating similar to a team with a mid to low lottery pick. The coach with the worst net rating before being fired was Paul Silas for the New Orleans Hornets in 2012. Paul Silas would be replaced by Mike Dunlap, who only lasted one season. The coach with the best net rating before being fired was Dwane Casey for the Toronto Raptors in 2018. Casey was replaced by Nurse, who won a championship with the Raptors in his first season.
After recording the net rating of each team the season before the new coach was hired, I recorded the net rating of the team in the first, second, and third year of the head coach. To see if the coach made his team better or worse, I calculated the one year change, two year change, and three year change of the team with the new head coach. I found this by subtracting the net rating of the team prior to the new coach from the first year, second year, and third year net rating. The best one year change was a tie between Steve Clifford of the Orlando Magic in the 2013–14 NBA season and Tom Thibodeau of the Chicago Bulls in the 2010–11 NBA season. The Magic had a net rating of -10 before Clifford arrived and a net rating of -0.2 in Clifford’s first season with the team, an improvement of 9.8. Thibodeau had the same improvement, going from a net rating of -1.8 before he arrived to a net rating of 8 in his first season. On the flip side, the worst one year change was -16.7 by Byron Scott of the Cleveland Cavaliers in the 2010–11 season. The Cavaliers had a net rating of 7.1 before Scott became the coach, but LeBron James left for the Miami Heat the following season and the Cavaliers had a net rating of -9.1 in Scott’s first season with the team.
The chart above shows box plots of new head coach net ratings for their first three seasons and the season prior to their hiring. The median net rating rises each year, giving a sign of slight improvement following a coaching change. Additionally, the 1st quartile of the net ratings after three years is about even with the median of the net ratings of the year before the new coach was hired, meaning that there is a good improvement is a coach lasts three years. The largest jump seems to be from two years after hiring to three years after hiring, meaning that the third year of a new head coach is when the biggest jump should be expected. The one outlier for the net rating after one year belongs to Steve Kerr for the Golden State Warriors in the 2014–15 season during his first title run.
The box plots of the changes in net rating after one, two, and three years give similar conclusions. The biggest jump being from year two to year three is also shown in these box plots. One interesting aspect shown by the plots above is that the one year change and two year change in net ratings seem to be very similar, with the only major difference being in their variances. The two year change varies slightly more than the one year change, but their centers are still very close to each other as they are both centered around 0. The two outliers for the one year change in net rating are Byron Scott for the Cavaliers in 2010–11 and Flip Saunders for the Timberwolves in 2014–15. They had one year changes in net ratings of -16.7 and -13, respectively. The outlier for the two year change in net rating is Paul Silas for the Hornets in 2011–12, when Silas had a two year change of -16.8 and was fired after having one of the worst coaching stints in NBA history.
The bar graph above gives the means of the values of the one year, two year, and three year change in net ratings, as well as the two year improvement of the coach. The two year improvement is just the difference between the three year change and the one year change. This stat gives an idea of how much the coach improved his team after he took over without accounting for the performance of the previous coach. The graph above has error bars, which give the upper and lower bounds of a 95% confidence interval. A 95% confidence interval is an interval in which we are 95% confident that the true mean lies in. The graph above shows that there is likely to be improvement for each subsequent year after a new head coach is hired, however only the three year change is statistically significant from 0. Additionally, the two year improvement is very, very close to being statistically significant above 0, meaning that a coach is likely to improve during his tenure with his team.
The final aspect of new head coaches that I looked at was whether they made the playoffs and how long it took them if they did. Of the 81 coaches included in the study, 36 of them never made the playoffs. That means that 44% of new head coaches never made the playoffs. Moreover, if the coach did make the playoffs, it was likely in his first two years. About 47% of the new head coaches made the playoffs within their first two years with their new team. The chance of remaining the coach after missing the playoffs in the first two years was only about 30%. Some conclusions that can be made from the playoff data is that if a new head coach does not make the playoffs within their first two years, it is likely that they will be fired.
The findings from how long it took new head coaches to make the playoffs caused me to look into how long each coach lasted. For this sample, I included the three coaches that have lasted over ten years (Gregg Popovich, Rick Carlisle, and Erik Spoelstra) in order to have all the NBA teams represented. The distribution is heavily right skewed with three outliers (the three coaches mentioned above). Most coaches last fewer than four years, with only about one third of coaches having a fourth year with their team. About half of the new head coaches were let go of after two years, making the first year even more important. Making the playoffs in the first year caused a much more likely chance of remaining with the team for an extended amount of time. For example, about 41% of coaches that made the playoffs in their first season lasted more than three years with their team, compared to just 23% of coaches that did not make the playoffs in their first season. Additionally, only 14% of the coaches that made the playoffs in year one were fired, whereas 33% of coaches that did not make the playoffs in their first year were fired.
Coaching changes happen every offseason in the NBA. Coaching is an often underrated aspect of NBA teams and can help to shape a playoff team into a contending team. Finding the right coach is an important task for every NBA franchise, but only three franchises (Heat, Mavericks, and Spurs) have truly had a reliable coach for a sustained amount of time. Many teams are likely to make a coaching change this year, including the Nets, 76ers, Rockets, and Bulls, and they have to evaluate their coaching candidates carefully in order to improve their teams for the foreseeable future. Having coaching stability provides sustained success for teams across several years, just as the Heat, Mavericks, and Spurs have had. Some teams may have found their coach of the future, such as Brad Stevens for the Celtics, Frank Vogel for the Lakers, Mike Malone for the Nuggets, and Nick Nurse for the Raptors. Other teams, however, have had an incredible amount of coach turnover and need to find some stability, such as the Cavaliers, Grizzlies, Suns, and Timberwolves. For coaches, year one is critical. A good year can give stability for the future while a bad year means that their time will come to an end soon. Most teams move on from a coach after one or two unsuccessful seasons in which they do not make the playoffs. However, if a team finds a coach they deem worthy of making it into year three, they should expect a large improvement as year two to year three is often the largest jump in net rating for a coach’s team. Knowing this, I would suggest that NBA teams should not be so quick to fire coaches after a couple of bad seasons and instead give them time for them to make the jump. In conclusion, coaching is an important part of an NBA team and can be the difference between a good team and a bad team.