How Should NBA Coaches Use Their Challenges?

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For the 2023-24 NBA season, the league adopted a new policy allowing coaches a second challenge if their first challenge results in a successful overturn. NBA coaches successfully challenge out-of-bounds calls at a 77.7% clip; they successfully challenge foul calls at a 53% rate, and on average, they are successful with 59.5% of their challenges.

But what is the optimal challenge strategy for an NBA coach?

Let’s start by looking at the expected value of challenging each of the likely borderline calls. Table 1 lists the expected value of challenging in each of the following game situations:

• Getting one extra possession

• Getting two free-throws

• Getting three free-throws

• Goaltending

Table 1: The value of challenging for each game situation

Challenging a 3-point foul, whether it’s your team committing the foul or getting fouled, has a high value proposition, worth 1.247 points. The next highest value play is challenging a goal-tending call, worth 1.19 points. The third highest value decision is to challenge an out-of-bounds play with the aim of getting an extra possession, worth 0.901 points. The least worthwhile challenge, based on league challenge success rates, is a 2-point foul challenge, worth only 0.831 points.

Now, let’s look at these two scenarios:

• Scenario 1: There is a challengeable play right now and there will be one more challengeable play later on in the game. The team has not used a challenge yet.

• Scenario 2: There is a challengeable play right now and there will be two more challengeable plays later on in the game. The team has not used a challenge yet.

Scenario 1

There is a challengeable play right now and there will be one more challengeable play later on in the game. The team has not used a challenge yet.

Assuming 60% of challengeable plays are out-of-bounds play, 30% are 2-point fouls, 5% are 3-point fouls, and 5% are goal-tending calls, the value of having an additional challenge is 0.912 points.

So, what does the breakeven challenge win rate have to be for this scenario to make the challenge worthwhile? Shown in Table 2, coaches should aggressively challenge borderline 3-point fouls and goaltending calls, have moderate aggression when challenging borderline 2-point fouls, and require more than a 44% overturn rate to challenge out-of-bounds calls in Scenario 1.

Table 2: Challenge overturn rate breakeven points for Scenario 1

Scenario 2

There is a challengeable play right now and there will be two more challengeable plays later on in the game. The team has not used a challenge yet.

The value of keeping two challenge opportunities is 1.241 points, so the bar for challenging is much higher in this scenario.

As evident from Table 3, coaches should challenge 3-point fouls and goaltending calls with moderate aggression, but need to have much more conviction before challenging 2-point fouls or out-of-bounds calls in Scenario 2.

Table 3: Challenge overturn rate breakeven points for Scenario 2

Conclusion

Early in games, when multiple additional challenge opportunities are likely to present themselves, coaches should have moderate aggression when challenging 3-point fouls and goaltending calls, but should have caution when challenging out-of-bounds plays and 2-point fouls.

Towards the middle of the game, when one more challengeable play is likely to present itself, coaches should be even more aggressive in challenging 3-point plays and goaltending calls, while also showing aggression in challenging 2-point plays. They should still have caution when challenging out-of-bounds plays.

Late in the game, when the current challengeable play is likely the last challengeable play of the game, coaches should challenge the play if they have a non-negligible chance at success.

Factors such as preventing an additional personal foul on your superstar player or tacking on an additional foul to your opponent’s superstar player may also be relevant in challenge decisions.