The Evolution of NBA Stats
By John Schuhmann
Every year, the NBA is getting smarter. Front offices, coaches, players and fans around the league are all learning more about the game by embracing analytics on some level. More than ever, we know which shots are good shots, which players can make their teams better without scoring, and the value of efficiency.
Now, we're taking another step forward with the relaunch of NBA.com/stats, a massive tool which will allow you to learn more about the league's teams and players, and see if the numbers back up what you see with your eyes.
If you're new to the advanced stats movement, we're here to catch you up. And the best place to begin is with pace, efficiency and the four factors.
Advanced stats on the team level really start with the acknowledgement of pace. Different teams play at different speeds and numbers should reflect that. So instead of measuring team offense or defense by the number of points scored or allowed per game, we look at them on a per-possession basis, putting every team on a level playing field.
For example, opponents of the Brooklyn Nets score just 94.7 points per game, the fifth-lowest mark in the league. But that number is more about the pace that the Nets play at than how good they are defensively.
The Nets and their opponents average just 90.5 possessions per 48 minutes, the slowest pace in the league. And that number helps keep their opponent points down. On a per-possession basis, the Nets actually rank 17th defensively.
In the same vein, the Nets are a better offensive team than their per-game numbers indicate. On a per-game basis, they score the 20th most points in the league. But when you adjust for pace, they have the 12th most efficient offense.
This is important, because when two teams play each other, they're going to each get the ball an equal number of times (give or take an extra possession or two, depending on how the ends of quarters go) and numbers that aren't pace-adjusted are kind of worthless.
Efficiency and the four factors
Sometimes, you'll hear a broadcaster reference field goal percentage as a way to measure how effective an offense or defense is. But that doesn't tell the whole story either.
For example, the Atlanta Hawks are a very good shooting team (sixth best in the league). But they also also turn the ball over too often, don't get to the free throw line much, and fail to grab many offensive rebounds. Overall, the Hawks rank 15th offensively.
Per-game numbers and standard field goal percentage both lack a lot of context. Offensive and defensive efficiency, however, are tidy numbers that tell you exactly how good teams are on either end of the floor.
To make efficiency resemble standard stats somewhat, we look at points scored or allowed per 100 possessions. And the formulas are simple...
Offensive efficiency (OffRtg) = 100 x (Points / Possessions)
Defensive efficiency (DefRtg) = 100 x (Opponent points / Opponent possessions)
Possessions do not necessarily end with a shot attempt. An offensive rebound extends a possession, which doesn't end until the other team gets the ball.
The number of possessions a team has over the course of a game (or the season, or a stretch of games) is estimated using standard boxscore stats and the following formula:
Possessions = FGA + (0.44 x FTA) + TO - OReb
At the All-Star break, the Miami Heat have been the best offensive team in the league, scoring 110.4 points per 100 possessions. The Indiana Pacers have been the best defensive team in the league, allowing just 95.9.
As noted in the paragraph about the Hawks above, there are four factors that affect efficiency on either end of the floor: Shooting, rebounding, turnovers and free throw rate. And if you truly want to be a strong offensive team, you have to be good at more than one of the four.
Here's how they break down.
Shooting (effective field goal percentage)
Effective field goal percentage (EFG%) is a better number than standard field goal percentage, because it takes the added value of a 3-pointer into account.
Here's the formula: EFG% = (FGM + (0.5 * 3PM)) / FGA
And here's an example of why EFG% is a better number than standard FG%:
Team A gets the ball 10 times, shoots 10 2-point shots, and makes five of them. So they had a field goal percentage of 50.0 percent and scored 10 points on those 10 shots.
Team B gets the ball 10 times, shoots 3-for-5 from 2-point range and 2-for-5 from 3-point range. They also had a field goal percentage of 50.0 percent, but scored 12 points on those 10 shots.
Team A's effective field goal percentage was 50,0 percent. Team B's was 60 percent.
Instead of looking at rebounds per game, we want to look at rebounding percentage, because other factors (how well a team or its opponent shoots and how often they turn it over) can affect raw rebounding numbers.
Here are our simple formulas on the team level:
Offensive rebounding percentage (OREB%) = Offensive rebounds / (Offensive rebounds + Opponent defensive rebounds)
Defensive rebounding percentage (DREB%) = Defensive rebounds / (Defensive rebounds + Opponent offensive rebounds)
And here's an example of how rebounding percentage can affect efficiency...
Team A gets the ball 10 times, shoots 10 2-point shots, makes five of them, and doesn't rebound any of the misses. So they scored 10 points on their 10 possessions.
Team B gets the ball 10 times, shoots 10 initial 2-point shots and makes only four of the 10. But Team B rebounds two of their six initial misses and follows them each up with a 2-point make.
So both teams shot 50 percent (whether you're looking at standard or effective field goal percentage). Team A shot 5-for-10 and Team B shot 6-for-12. But Team B was more efficient, scoring 12 points on its 10 possessions, while Team A scored just 10.
Team A had an offensive rebounding percentage (OREB%) of 0.0 percent, because they grabbed zero of the five available offensive boards. Team B had an OREB% of 33.3 percent, because they grabbed two of the six available offensive boards.
At the All-Star break, the Memphis Grizzlies are the best offensive rebounding team in the league, grabbing 32.0 percent of available offensive boards. And the New York Knicks are the best defensive rebounding team in the league, allowing their opponents to grab only 24.8 percent of available offensive rebounds.
You can't score if you aren't able to actually get a shot. Turnovers are obviously critical, because they wipe out an offensive possession entirely.
Here's your turnover rate example...
Team A gets the ball 10 times, doesn't turn the ball over at all, and, once again, goes 5-for-10 on 2-point shots with no offensive rebounds.
Team B gets the ball 10 times, turns the ball over twice, and shoots 4-for-8 on 2-point shots, without any offensive rebounds.
Again, both teams shot 50 percent (FG% or EFG%). But Team A was more efficient, because it scored 10 points on its 10 possessions, while Team B scored just eight.
Team A had a turnover rate (TmTOV%) - turnovers per 100 possessions - of 0.0. Team B had a turnover rate of 20.0.
At the All-Star break, the Knicks have done the best job of taking care of the ball, committing just 12.6 turnovers per 100 possessions. And the Los Angeles Clippers lead the league by forcing 17.7 turnovers per 100 possessions.
Simply, a trip to the line for two free throws is worth more than a shot from the field. On average, two free throws are worth 1.56 points (because the league shoots 75.3 percent from the line), while a shot from the field is worth 0.99 (because the league-average EFG% is 49.3 percent).
One more time: Team A gets the ball 10 times and shoots 5-for-10 on 2-point shots, with no turnovers, offensive rebounds or free throws.
Team B gets the ball 10 times, shots 4-for-8 on 2-point shots, but gets fouled on two other attempts and shoots 3-for-4 from the free throw line.
Once more, both teams shot 50 percent (FG% or EFG%). But Team B was more efficient, scoring 11 points on its 10 possessions, while Team A scored just 10.
Free throw rate (FTA rate) is measured by a simple formula: FTA/FGA. Team A had a FTA Rate of .000 (0/10). Team B had a free throw rate of .500 (4/8).
At the All-Star break, the Oklahoma City Thunder lead the league in free throw rate, attempting 34 free throws for every 100 field goal attempts. The San Antonio Spurs do the best job of keeping their opponents off the line. Their opponents have attempted just 23 free throws per 100 field goal attempts.
So those are the basics of team efficiency. And as we evaluate players, we need to look beyond their boxscore stats and focus on how they affect their team's success on either end of the floor.
We'll do just that in another story.