What Is A Possession?
{See the Updated Changed Position on Definition of Possession Here]

A possession is any identifiable scoring opportunity. A possession begins with the first possession of a half, a rebound, an opponent's score, or an opponent's turnover. A possession also begins with a technical foul on the possessory team.  A possession ends with a shot, a free throw opportunity, or a turnover. Occasionally, a possession may begin but not end. For example, a team may start a possession near the end of a period and hold the ball without a possession-ending event before the period closes.

Should a possession count when it begins or when it ends?

I find two compelling reasons to count a possession only when it ends. First, the team in possession must act to end the possession.  Second, a team with a game ending possession and a lead should hold the ball until time expires if possible. If this final action is counted as a possession, the team's average efficiency for that game falls slightly because the team has an extra possession even though the team does not even attempt to score.

For ease of use, I have developed the following algorithm to compute the number of possessions.

Total Possessions = {Total shots + total turnovers + (total free throws)/2}.

When a team's total free throw attempts is an odd number, I round up to the next higher even number. Obviously, this treatment of free throws is approximate because some free throw opportunities only have one shot, yet others have 3. Rather than precisely track these free throw possessions, there is a relatively small error in the two shot per possession and the round-up protocols. As I watch games and track possessions live I do count the actually number of free throw opportunities without the simplifying assumption.

The last issue that must be addressed when discussing possessions is whether an offensive rebound is the continuation of the possessions or the beginning of a new possession.  While there appears to be uniformity with respect to counting a possession when it ends rather than begins, there is some disagreement about how to treat an offensive rebound.  When I began my analysis in the early 1990s, I resolved this issue by electing to define an offensive rebound, just as a defensive rebound, as the start of a new possession, e.g. a new opportunity to score.  When a team elects to take a shot, it ends its possession, per the discussion above and it relinquishes its possession of the ball.  If the shot is successful, the oppponent will automatically begin its next possession, and if the shot is errant, the ball is "live" and which ever team can gain possession via the rebound will begin a new possession. 

This definition of a possession is consistent with the concept that a possession ends with a shot, and consistent with the idea that a possession is an opportunity to score.  Not all who consider this question arrive at the same answer as I have.  Most notably, Ken Pomeroy does not start a new possession with the occurrence of an offensive rebound.  Those who prefer Pomeroy's definition rely primarily upon the apparent impact that this definitional feature has on calculations of game pace, e.g. number of possessions per game.  Pomeroy's system produces consistently lower numbers of possessions than does my system, and that difference is the number of offensive rebounds that each team obtains in a game.

However, this is a distinction without a difference because when an analyst uses either system to compare pace, team to team, game to game, or year to year, the relative relationships remain very similar if not the same.  With the Pomeroy system, the NCAA D1 average game pace is about 71 possessions per game and with my system, the NCAA D1 average game pace is about 81 possessions per game.  However, Teams X, Y, and Z that average 65, 71, and 77 possessions per game, per Pomeroy would average 75, 81, and 87 possessions per game per my system.

Your comments are always welcome.



What Is Basketball?

What is a Possession?

Change in Position on Definition of Possessions

What Is Net Game Efficiency?

Why Do "Upsets" Occur?


Do Objective Performance Measures Like NGE
Account For Intangible?


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