Methodology

Possessions

Possessions are the backbone of modern NBA analytics and it can be helpful to understand how they are defined. An NBA possession starts when a team gains possession of the ball and ends when they lose possession.

Possession ends

A possession can end in a number of ways:

• A turnover (bad pass, travel, offensive foul, shot clock violation, etc)
• A missed shot resulting in a defensive rebound
• A made field goal
• A made free throw from the last attempt of a set of free throws (1, 2 or 3 shots)
• The end of a period (regulation or overtime)
• A jump ball in which the defensive team secures possession

Long possessions

A team can have multiple shots in a single possession if they gain an offensive rebound. A timeout could be called in the middle of a possession and multiple substitutions could be made within a single possession if there are sufficient stoppages in play.

In some circumstances, a team can score multiple times on the same possession, for instance when a flagrant foul is called on a made basket, the team retains possession of the ball after penalty free throws and has another opportunity to score on the same possession.

Technical fouls

Importantly, a technical foul does not end a possession. A team can be called for a technical foul and mid-possession, the opposing team shoots free throws on their own basket, before the ball is returned to the offensive team for the resumption of the possession.

Because of this, points from technical fouls are commonly excluded from possession efficiency calculations (since they would tend to inflate the efficiency of the team that made lots of technical free throws).

Counting methodology

At present we count possessions from the NBA's play-by-play data in which each play contains the team ID of the team currently in possession of the ball. Prior to the 2020-21 season this rich play-by-play data is unavailable so for these seasons we rely on Dean Oliver's method of calculating possessions using raw box score data. This is a remarkably accurate, though inexact approximation of possessions. The numbers from these calculations can commonly be found on Basketball Reference but these numbers differ from the more accurate numbers most modern NBA statistics sites use (such as Cleaning the Glass, databallr, Dunks and Threes, etc).

Next up: Pace