There are numerous official statistics kept by the NBA, and another entire batch of metrics in the realm of advanced NBA analytics. The bottom line in pro basketball is winning games, but which of these statistics are most important? And once that is determined, just how important is each statistic?
In an attempt to answer the above questions, Dean Oliver, author of the book Basketball on Paper, took a hard look at NBA statistics and how they translate to winning or losing games. He developed a concept known as the Four Factors.
How the Four Factors are Calculated
Let's break down the Four Factors individually. For shooting, we use Effective Field Goal Percentage as our metric. This metric is similar to Field Goal Percentage, but adds an additional parameter. This parameter adjusts for the fact that 3-point field goals are worth 50 percent more than 2-point field goals. So, this gives us a more accurate picture of productivity than we get with standard Field Goal Percentage.
For turnovers, Turnover Percentage gives us the clearest picture of effectiveness (or ineffectiveness). This metric estimates the number of turnovers a team commits per 100 possessions. Since this value is calculated on a per-possession basis rather than a per-game basis, this is a more meaningful statistic than standard boxscore turnover numbers.
Rebounding is represented by Rebound Percentage, which estimates the percentage of total available rebounds a team grabs during a given period of time, or number of games. Once again, this is a step up from the official NBA rebounding totals and averages that are more commonly used.
Lastly, we have free throws. We want to see how often a team gets to the free throw line, versus how many possessions end in a shot attempt. Free Throw Rate accomplishes this by dividing free throw attempts by field goal attempts, thus giving us a ratio that tells us how effective a team is at getting fouled and accumulating free throw tries.
Incidentally, all four factors can be calculated for offense and defense, so in essence, there are eight total factors we can look at.
How to Interpret the Four Factors
The Four Factors are most meaningful when they can be used to compare teams to each other in the same statistical categories. For example, if we know a team's Effective Field Goal Percentage is 52.7, that's nice, but until we know how other teams are performing in the same category, it doesn't mean much.
To shed further light on this, let's examine some league averages from a recent NBA season:
- Effective Field Goal Percentage: 48.7
- Turnover Percentage: 13.8
- Offensive Rebound Percentage: 27.0
- Defensive Rebound Percentage: 73.0
- Free Throw Rate: 20.8
Now that we have these league averages, we can not only compare one team to another, but we can also determine which teams are good, bad or average in each statistical category.
Benefits of the Four Factors
It's always helpful to boil down a ton of statistical information to just a few key categories, and the Four Factors concept accomplishes this. Based on Dean Oliver's research, these are the key contributing factors toward winning in the NBA. We can therefore use this data to compare teams and see where their strengths and weaknesses lie. This is a very useful tool to see where teams need improvement, and where they have advantages over their opponents.
Negatives of the Four Factors
One thing the Four Factors concept is not intended to be used for is determining a "formula" that teams need to follow in order to be successful. Much like some people think a team must have a dominant center to win championships, others look at the Four Factors and assert that a team needs to perform at a certain level in shooting, for example. Balance is always a plus, but there is no minimum level of performance a team needs to maintain in each category to win games.
Many times, teams are out-of-balance. The key is do they do enough good things to overcome the things they don't do well...and this can come in any combination with regard to statistical categories. Going back to the dominant center example above: if a team has Michael Jordan and Scottie Pippen, they don't need a dominant center to win multiple championships - the 1990s Chicago Bulls certainly proved that!
The Four Factors - Behind the Number
As stated above, the Four Factors consist of shooting (Effective Field Goal Percentage), turnovers (Turnover Percentage), rebounding (Rebound Percentage) and free throws (Free Throw Rate). Recall that each of these metrics has an offensive and defensive component (the only formula that differs for defense is Rebound Percentage). These metrics are calculated as follows:
- Effective Field Goal Percentage = (field goals made + 0.5 * 3-point field goals made) / field goals attempted
- Turnover Percentage = 100 * turnovers / (field goal attempts + 0.44 * free throw attempts + turnovers)
- Rebound Percentage (offensive) = offensive rebounds / ( offensive rebounds + opponents' defensive rebounds)
- Free Throw Rate = free throw attempts / field goal attempts
Dean Oliver also assigned weights to each category, to give each one the appropriate amount of "importance". The values he uses are: shooting - 40 percent, turnovers - 25 percent, rebounding - 20 percent and free throws - 15 percent.
Interestingly, these weights could be used to construct a single-number team rating formula, but one doesn't often see that. This is likely because the point of the Four Factors is to look specifically at each statistical category and make comparisons...coming up with an overall rating using all the factors does not help us do that. However, that is one additional use this concept has.
Before advanced statistics were used in the NBA, team executives used rather crude methods to evaluate their own teams, and their opponents as well. Some might look at per-game averages and decide, "We need more rebounding." Some would just try to increase the overall talent level on their clubs any way they could, and some would just use experience and "gut feelings" to make judgments - most didn't use statistics of any kind in their decision-making process.
Now, with things like the Four Factors in hand, even "armchair" general managers can take a pretty sophisticated look at the teams they follow, and make sound evaluations on how the various teams in the league stack up.