Era-Adjusted Model

The NBA is a constantly adjusting league, undergoing trends as teams strategize to counter recent developments from one another. For example, there’s often a give and take between offensive rebounding and transition defense, with some coaches opting to avoid crashing the glass for second-chance opportunities in favor of getting back to prevent easy buckets.

In a vacuum, the Four Factors originally developed by Dean Oliver—effective field-goal percentage (shooting), turnover percentage (turnovers), rebounding percentage (rebounding) and free throws per field goal attempt (fouling)—can be rather telling. They break down success or failure in the core components of the sport.

But we can make them mean even more in a historical contest by adjusting for the era.

The adjusted factors on each side of the court are rather easy to calculate. If a higher score is desired, divide the actual result by the league average during the year in question, then multiply by 100. A resulting mark of 100 indicates perfect averageness, while anything higher is a positive. If a lower score is the goal, divide the league average by the actual result, then multiply by 100. The scale of the results still applies in the same manner.

The benefit of this process is significant.

To show it, let’s take two teams with some of the best effective field-goal percentages of all time—the 1998-99 Houston Rockets (50.6 eFG%) and the 1983-84 Los Angeles Lakers (53.6). In a vacuum, the Lakers are clearly superior, but they also had the luxury of playing during a time in which it was far easier to shoot a higher percentage. The Rockets played during a lockout-shortened season that produced a remarkably low league average of 46.6 and was sandwiched between two campaigns at 47.8.

When we adjust for the eras, the Rockets (108.58 adjusted eFG%) jump ahead of the Lakers (108.28). Given the ever-changing context of the NBA, they were the better shooting team, even if the Lakers were more efficient on a surface level.

We can also do this with teams’ overall offensive and defensive ratings by following the exact same processes illustrated above.

Beyond that, we can calculate Team Rating by averaging the adjusted offensive and defensive scores. A Team Rating of exactly 100 indicates that a squad was perfectly average—whether by being perfectly average on both ends or by being as bad on one as it was good on the other.