Boost in NBA Challenges Rewards Celtics’ Mazzulla: Data Viz

An average of 1.01 times per game this season, NBA fans have been treated to the thrill of watching everyone stand around as the referees walk over to the scorer’s table to review a coach’s challenge. This ritual was performed just 0.65 times per game last season.

The increase is due to the fact that, for the first time since the NBA introduced the coach’s challenge in 2019, coaches are now awarded a second challenge if their first is successful. Previously, coaches were only allowed to challenge once, even if they got it right.

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Coaches now have more opportunities to impact the game via challenging calls, which raises the question: Who is most effective at it?

In terms of successful challenges per game, no coach has been better than the Boston Celtics’ Joe Mazzulla. In approximately a season and a half since replacing Ime Udoka, he has gotten an average of 0.31 calls overturned per game, surpassing all his peers. The Oklahoma City Thunder’s Mark Daigneault trails closely behind at 0.27, followed by the Utah Jazz’s Will Hardy (0.25) and the Dallas Mavericks’ Jason Kidd (0.24).

Leading all coaches with a success rate of 69%, Mazzulla’s strength is his accuracy. After going 17 for 24 on challenges in his first season, he’s already 21 for 31 as of Jan. 14 this year. Among coaches with a sample size spanning multiple full seasons, the Memphis Grizzlies’ Taylor Jenkins comes in first at 67%.

Daigneault is more of a volume shooter. His 54% challenge success rate is still good—league average historically is around 50%—but his frequency nears the top of the league. He has used 0.51 challenges per game, third behind only Nick Nurse (0.54) and Mike Brown (0.55), who are all about getting attempts up—the Carmelo Anthonys of challenging, so to speak.

For the past four years, there has been a clear divide between coaches willing to use challenges early in games versus those preferring to save them for crunch time. In the 2022-23 season, for instance, Daigneault used 21 challenges prior to the fourth quarter while Mazzulla only used one. The Miami Heat’s Erik Spoelstra never challenged prior to the final period all last year.

The new rule mitigates the drawbacks of early challenges, prompting previously hesitant coaches to pull the trigger. First-quarter challenges are up 194% so far in 2023-24, while second- and third-quarter challenges are also up 123% and 81%, respectively.

Mazzulla’s approach has changed the most dramatically. In addition to being the most astute at assessing potential challenge situations, he has also proven to be adaptable in his tactics. He has already used a whopping 21 challenges in the first three periods of games this season.

Using more challenges early in games has helped Mazzulla boost his volume—Daigneault was the overall leader in successful challenges per game prior to this season—but is this just the coaching equivalent of stat-padding? Could saving challenges for end game scenarios be a more winning strategy?

To reach a conclusion, we analyzed every challenge in the 2022-23 season from the five coaches who most frequently challenged prior to the fourth quarter and the five coaches who were most hesitant to do so. We used Inpredictable’s Win Probability Calculator to determine the increase in win probability from every successful challenge, and then added those instances together to get an estimate of the expected wins added via challenges over the course of the season for each coach.

It’s worth noting that the calculator tool only takes into account the score, possession and time remaining, so not all variables are captured in our methodology. For example, overturning an early foul call against a star player reduces the chance that that player will eventually foul out. On the flip side, losing that challenge would cost the team a potentially valuable timeout.

The results do not show a significant edge under the old rulebook for either of these diametrically opposed strategies. If anything, the data favor being proactive. The five coaches who were most trigger-happy with challenges combined for 4.06 expected wins added for their teams, slightly more than the 3.71 totaled by the five coaches who were most inclined to save their challenges.

Assessing individual coaches based on this metric may not be instructive, however, because those who added the most expected wins did so mostly by converting a small number of very high-leverage, late-game challenges. Whether or not coaches get those particular opportunities isn’t in their control.

For instance, the Portland Trail BlazersChauncey Billups only went 10 for 25 on challenges last year, as opposed to 17 for 24 by Mazzulla, but the former added 1.18 expected wins, compared to only 0.77 for the latter. Mazzulla didn’t have a single challenge that increased his team’s win probability by more than 15%, but Billups flipped one defensive foul to an offensive foul with 11 seconds left in a game that increased the Blazers’ chances of winning that contest by 39.7%. That alone was more win probability added than some coaches accumulated throughout the entire season.

Indeed, aside from a few game-swinging outliers, the impact of challenges overall is relatively small. On average, the best coaches are only expected to flip the outcome of one game out of 82, compared to a coach who never challenges at all.

Still, if a coach can give his team a better chance to secure that one additional win, then you want the best on your sideline. Right now that’s Mazzulla.

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